Potassium compound and positive electrode active material for potassium ion secondary batteries containing same

ABSTRACT

Provided is a material that can be used as a potassium secondary battery positive electrode active material (particularly a potassium ion secondary battery positive electrode active material), other than Prussian blue, by using a potassium compound and a potassium ion secondary battery positive electrode active material comprising the potassium compound, the potassium compound being represented by general formula (1):
 
K n A k BO m ,
 
wherein A is a positive divalent element in groups 7 to 11 of the periodic table; B is positive tetravalent silicon, germanium, titanium or manganese, excluding a case in which A is manganese and B is titanium, and a case in which A is cobalt and B is silicon; n is 1.5 to 2.5; and m is 3.5 to 4.5.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/JP2016/086415, filed Dec. 7, 2016, which claims the benefit ofpriority of Japanese Patent Application No. 2015-238,898, filed Dec. 7,2015, the contents of both being incorporated by reference in theirentirety for all purposes.

TECHNICAL FIELD

The present invention relates to a potassium compound and a potassiumion secondary battery positive electrode active material comprising thesame.

BACKGROUND ART

Research and development of secondary batteries etc. using lithium ions,sodium ions, magnesium ions, aluminum ions, or the like as carrier ionshave been recently drawing attention. From the viewpoint of resource andcost advantages as well as adaptability etc. of a wide variety of novelmaterials, secondary batteries using other carrier ions have beenstudied and developed as an actual solution for next-generation,large-sized storage batteries.

Among these, potassium ions, which have a low Lewis acidity and a smallCoulomb interaction, have been expected to produce ultrafast chargeableand dischargeable storage batteries. Further, because of the largeatomic weight of potassium, potassium-containing materials have a highertrue density than lithium-containing materials, and an improvement inenergy density per volume can be expected. Moreover, even when dendritesoccur in potassium and cause a short circuit, since they arespontaneously dissolved by generated heat and easily eliminate the shortcircuit, thermal runway is unlikely to occur. Additionally, sincepotassium does not form an alloy with aluminum, there is no need to useexpensive copper as a current collector of the negative electrode side.

Such potassium ion secondary batteries are new subjects of study and arehighly expected to be novel storage batteries that will create newindustries. As such a potassium ion secondary battery, a potassiumsecondary battery in which Prussian blue (KCuFe(CN)₆), which is used asa positive electrode active material, is combined with carbon, which isused as a negative electrode material, has been reported, wherein a highdischarge voltage of 3.8 V is attained (NPL 1).

CITATION LIST Non-Patent Literature

-   NPL 1: C. D. Wessells et al., Nat. Comm., 2, 550 (2011)-   NPL 2: Komaba et al., Electrochem. Commun., 60, 172-175 (2015)

SUMMARY OF INVENTION Technical Problem

However, since potassium has a large atomic weight and a large ionicradius, the theoretical capacity of a potassium ion secondary battery islikely to be lower than that of a lithium ion secondary battery. Sb,KC₈, graphite, etc., are known as negative electrode materials ofpotassium ion secondary batteries (NPL 2); however, positive electrodeactive materials comprising Prussian blue have a low capacity(theoretical charge capacity: about 85 mAhg⁻¹; effective capacity: 65mAhg⁻¹ (NPL 1)). Other than Prussian blue, effective positive electrodeactive materials have not been found. An object of the present inventionis to provide a material other than Prussian blue, the material havinghigh theoretical charge-discharge capacity and being able to be used asa potassium ion secondary battery positive electrode active material.

Solution to Problem

The inventors conducted extensive research to solve the above problems.As a result, they found that a potassium compound having a specificcomposition enables insertion and extraction of potassium ions, and hashigh theoretical charge-discharge capacity so that it can be used as apotassium ion secondary battery positive electrode active material.Based on this findings, the inventors conducted further research andaccomplished the present invention. Specifically, the present inventionincludes the following.

-   Item 1. A potassium compound represented by general formula (1):    K_(n)A_(k)BO_(m),    wherein A is a positive divalent element in groups 7 to 11 of the    periodic table; B is positive tetravalent silicon, germanium,    titanium or manganese, excluding a case in which A is manganese and    B is titanium, and a case in which A is cobalt and B is silicon; k    is 0.6 to 1.5; n is 1.5 to 2.5; and m is 3.5 to 4.5.-   Item 2. The potassium compound according to Item 1, wherein A is    manganese, iron, cobalt, nickel, or copper.-   Item 3. The potassium compound according to Item 1 or 2, wherein the    potassium compound has at least one member selected from the group    consisting of a cubic structure, a tetragonal structure, an    orthorhombic structure, and a monoclinic structure.-   Item 4. The potassium compound according to any one of Items 1 to 3,    wherein the potassium compound has a mean particle diameter of 0.2    to 200 μm.-   Item 5. A method for producing the potassium compound according to    any one of Items 1 to 4, the method comprising a heating step of    heating a mixture containing potassium, an element in groups 7 to 11    of the periodic table, silicon, germanium, titanium or manganese,    and oxygen.-   Item 6. The production method according to Item 5, wherein the    heating temperature in the heating step is 600 to 1500° C.-   Item 7. A potassium ion secondary battery positive electrode active    material comprising a potassium compound represented by general    formula (2): K_(n)A_(k)BO_(m),    wherein A is a positive divalent element in groups 7 to 11 of the    periodic table; B is positive tetravalent silicon, germanium,    titanium or manganese; k is 0.6 to 1.5; n is 1.5 to 2.5; and m is    3.5 to 4.5.-   Item 8. A potassium ion secondary battery positive electrode active    material comprising the potassium compound according to any one of    Items 1 to 4.-   Item 9. A potassium ion secondary battery positive electrode    comprising the potassium ion secondary battery positive electrode    active material according to Item 7 or 8.-   Item 10. The potassium ion secondary battery positive electrode    according to Item 9, further comprising a conductive material.-   Item 11. A potassium ion secondary battery comprising the potassium    ion secondary battery positive electrode according to Item 9 or 10.

Advantageous Effects of Invention

The potassium compound of the present invention enables insertion andextraction of potassium ions, and thus can be used as a potassium ionsecondary battery positive electrode active material. In particular,potassium ion secondary batteries using the potassium compound of thepresent invention as a positive electrode active material are expectedto have high capacity and high potential.

Further, since potassium does not form an alloy with aluminum, apotassium ion secondary battery using the potassium compound of thepresent invention as a positive electrode active material can also uselow-cost aluminum rather than expensive copper as a negative electrodecurrent collector.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows X-ray diffraction patterns of K₂Fe²⁺Si⁴⁺O₄ obtained inExample 1-2.

FIG. 2 shows X-ray diffraction patterns of K₂Fe²⁺Si⁴⁺O₄ obtained inExample 1-3.

FIG. 3 shows the results of comparing the X-ray diffraction pattern ofK₂Fe²⁺Si⁴⁺O₄ obtained in Example 1-2 with those of other iron silicatecompounds.

FIG. 4 shows X-ray diffraction patterns of K₂Fe²⁺Si⁴⁺O₄ obtained inExample 1-2 determined by powder X-ray diffraction.

FIG. 5 shows an SEM image of K₂Fe²⁺Si⁴⁺O₄ obtained in Example 1-2.

FIG. 6 shows the results of thermal stability measurement by TG-DTA ofK₂Fe²⁺Si⁴⁺O₄ obtained in Example 1-2.

FIG. 7 shows X-ray diffraction patterns of K₂Fe²⁺Si⁴⁺O₄ obtained inExample 1-2, K₂Fe²⁺Ge⁴⁺O₄ obtained in Example 2-1, K₂Fe²⁺Ti⁴⁺O₄ obtainedin Example 3-1, and K₂Fe²⁺Mn⁴⁺O₄ obtained in Example 4-1.

FIG. 8 shows a Rietveld X-ray diffraction pattern of K₂Fe²⁺Si⁴⁺O₄obtained in Example 1-2.

FIG. 9 shows X-ray diffraction patterns of K₂Fe²⁺Si⁴⁺O₄ obtained inExample 1-2, K₂Co²⁺Si⁴⁺O₄ obtained in Example 5-1, K₂Ni²⁺Si⁴⁺O₄ obtainedin Example 8-2, and K₂Mn²⁺Si⁴⁺O₄ obtained in Example 10-1.

FIG. 10 shows X-ray diffraction patterns of K₂Fe²⁺Si⁴⁺O₄ obtained inExample 1-2, K₂Fe²⁺Ge⁴⁺O₄ obtained in Example 2-1, K₂Fe²⁺Ti⁴⁺O₄ obtainedin Example 3-1, and K₂Fe²⁺Mn⁴⁺O₄ obtained in Example 4-3.

FIG. 11 shows SEM images of K₂Fe²⁺Si⁴⁺O₄ obtained in Example 1-2.

FIG. 12 shows a high-resolution SEM image of K₂Fe²⁺Si⁴⁺O₄ obtained inExample 1-2.

FIG. 13 shows a high-resolution SEM image of K₂Fe²⁺Si⁴⁺O₄ obtained inExample 1-2.

FIG. 14 shows the thermal stability and color tone after heating ofK₂Fe²⁺Si⁴⁺O₄ obtained in Example 1-2, K₂Co²⁺Si⁴⁺O₄ obtained in Example5-1, and K₂Mn²⁺Si⁴⁺O₄ obtained in Example 10-1. K₂Mn²⁺Si⁴⁺O₄ (purplecrystals), K₂Fe²⁺Si⁴⁺O₄ (yellowish-brown crystals), and K₂Co²⁺Si⁴⁺O₄(dark green crystals) are shown in ascending order of weight loss.

FIG. 15 shows SEM images of K₂Fe²⁺Ge⁴⁺O₄ obtained in Example 2-1.

FIG. 16 shows a high-resolution SEM image of K₂Fe²⁺Ge⁴⁺O₄ obtained inExample 2-1.

FIG. 17 shows SEM images of K₂Fe²⁺Ti⁴⁺O₄ obtained in Example 3-1.

FIG. 18 shows a high-resolution SEM image of K₂Fe²⁺Ti⁴⁺O₄ obtained inExample 3-1.

FIG. 19 shows X-ray diffraction patterns of K₂Fe²⁺Mn⁴⁺O₄ obtained inExample 4-3.

FIG. 20 show X-ray diffraction patterns (800° C.) of K₂Fe²⁺Mn⁴⁺O₄obtained in Example 4-3, K₂Mn²⁺Mn⁴⁺O₄ obtained in Example 13,K₂Ni²⁺Mn⁴⁺O₄ obtained in Example 14-1, and K₂Co²⁺Mn⁴⁺O₄ obtained inExample 15-1.

FIG. 21 shows X-ray diffraction patterns (1000° C.) of K₂Fe²⁺Mn⁴⁺O₄obtained in Example 4-3, K₂Mn²⁺Mn⁴⁺O₄ obtained in Example 13,K₂Ni²⁺Mn⁴⁺O₄ obtained in Example 14-1, and K₂Co²⁺Mn⁴⁺O₄ obtained inExample 15-1.

FIG. 22 shows an SEM image of K₂Fe²⁺Mn⁴⁺O₄ obtained in Example 4-3.

FIG. 23 shows SEM images of K₂Fe²⁺Mn⁴⁺O₄ obtained in Example 4-1.

FIG. 24 shows a high-resolution SEM image of K₂Fe²⁺Mn⁴⁺O₄ obtained inExample 4-1.

FIG. 25 shows X-ray diffraction patterns of K₂Co²⁺Si⁴⁺O₄ obtained inExamples 5-1 and 5-2.

FIG. 26 shows an SEM image of K₂Co²⁺Si⁴⁺O₄ obtained in Example 5-1.

FIG. 27 shows the results of thermal stability measurement by TG-DTA ofK₂Co²⁺Si⁴⁺O₄ obtained in Example 5-1.

FIG. 28 shows X-ray diffraction patterns of K₂Co²⁺Si⁴⁺O₄ obtained inExample 5-1, K₂Co²⁺Ge⁴⁺O₄ obtained in Example 6-1, and K₂Co²⁺Ti⁴⁺O₄obtained in Example 7-1.

FIG. 29 shows an SEM image of K₂Co²⁺Si⁴⁺O₄ obtained in Example 5-1.

FIG. 30 shows a high-resolution SEM image of K₂Co²⁺Si⁴⁺O₄ obtained inExample 5-1.

FIG. 31 shows SEM images of K₂Co²⁺Ge⁴⁺O₄ obtained in Example 6-1.

FIG. 32 shows high-resolution SEM images of K₂Co²⁺Ge⁴⁺O₄ obtained inExample 6-1.

FIG. 33 shows a high-resolution SEM image of K₂Co²⁺Ti⁴⁺O₄ obtained inExample 7-1.

FIG. 34 shows an SEM image of K₂Ni²⁺Si⁴⁺O₄ obtained in Example 8-1.

FIG. 35 shows X-ray diffraction patterns of K₂Ni²⁺Si⁴⁺O₄ obtained inExample 8-2.

FIG. 36 shows the results of thermal stability measurement by TG-DTA ofK₂Ni²⁺Si⁴⁺O₄ obtained in Example 8-2.

FIG. 37 shows SEM images of K₂Ni²⁺Si⁴⁺O₄ obtained in Example 8-2.

FIG. 38 shows a high-resolution SEM image of K₂Ni²⁺Si⁴⁺O₄ obtained inExample 8-2.

FIG. 39 shows X-ray diffraction patterns of K₂Cu²⁺Si⁴⁺O₄ obtained inExamples 11-1 and 11-2.

FIG. 40 shows X-ray diffraction patterns of K₂Mn²⁺Si⁴⁺O₄ obtained inExample 10-2.

FIG. 41 shows an SEM image of K₂Mn²⁺Si⁴⁺O₄ obtained in Example 10-1.

FIG. 42 shows the results of thermal stability measurement by TG-DTA ofK₂Mn²⁺Si⁴⁺O₄ obtained in Example 10-1.

FIG. 43 shows a Rietveld X-ray diffraction pattern of K₂Mn²⁺Si⁴⁺O₄obtained in Example 10-1.

FIG. 44 shows SEM images of K₂Mn²⁺Si⁴⁺O₄ obtained in Example 10-1.

FIG. 45 shows a high-resolution SEM image of K₂Mn²⁺Si⁴⁺O₄ obtained inExample 10-1.

FIG. 46 shows a high-resolution SEM image of K₂Mn²⁺Si⁴⁺O₄ obtained inExample 10-1.

FIG. 47 shows SEM images of K₂Mn²⁺Ge⁴⁺O₄ obtained in Example 12-1.

FIG. 48 shows a high-resolution SEM image of K₂Mn²⁺Ge⁴⁺O₄ obtained inExample 12-1.

FIG. 49 shows the results of SEM-EDX of K₂Mn²⁺Ge⁴⁺O₄ obtained in Example12-1.

FIG. 50 shows an SEM image of K₂Mn²⁺Mn⁴⁺O₄ obtained in Example 13.

FIG. 51 shows a high-resolution SEM image of K₂Mn²⁺Mn⁴⁺O₄ obtained inExample 13.

FIG. 52 shows a Rietveld X-ray diffraction pattern of K₂Mn²⁺Mn⁴⁺O₄obtained in Example 13.

FIG. 53 shows the results of SEM-EDX of K₂Mn²⁺Mn⁴⁺O₄ obtained in Example13.

FIG. 54 shows HAADF-STEM images of K₂Mn²⁺Mn⁴⁺O₄ obtained in Example 13.

FIG. 55 shows an SEM image of K₂Ni²⁺Mn⁴⁺O₄ obtained in Example 14.

FIG. 56 shows a high-resolution SEM image of K₂Ni²⁺Mn⁴⁺O₄ obtained inExample 14.

FIG. 57 shows the results of SEM-EDX of K₂Ni²⁺Mn⁴⁺O₄ obtained in Example14.

FIG. 58 shows a Rietveld X-ray diffraction pattern of K₂Co²⁺Mn⁴⁺O₄obtained in Example 15.

FIG. 59 shows an SEM image of K₂Co²⁺Mn⁴⁺O₄ obtained in Example 15.

FIG. 60 shows high-resolution SEM images of K₂Co²⁺Mn⁴⁺O₄ obtained inExample 15.

FIG. 61 shows the results of SEM-EDX of K₂Co²⁺Mn⁴⁺O₄ obtained in Example15.

FIG. 62 shows charge-discharge curves of a potassium half cell usingK₂Mn²⁺Mn⁴⁺O₄ obtained in Example 13.

FIG. 63 shows a sectional view of the test cell of Test Example 3.

FIG. 64 shows charge-discharge curves of a test cell using K₂Fe²⁺Mn⁴⁺O₄.

FIG. 65 shows charge-discharge curves of a test cell using K₂Ni²⁺Mn⁴⁺O₄.

FIG. 66 shows charge-discharge curves of a test cell using K₂Co2+Mn4+0₄.

FIG. 67 shows X-ray diffraction patterns of K₂Fe²⁺Mn⁴⁺O₄ before andafter charge-discharge test.

DESCRIPTION OF EMBODIMENTS

1. Potassium Compound

The potassium compound of the present invention is a potassium compoundrepresented by general formula (1): K_(n)A_(k)BO_(m), wherein A is apositive divalent element in groups 7 to 11 of the periodic table; B ispositive tetravalent silicon, germanium, titanium or manganese,excluding a case in which A is manganese and B is titanium, and a casein which A is cobalt and B is silicon; k is 0.6 to 1.5; n is 1.5 to 2.5;and m is 3.5 to 4.5 (hereinafter also referred to as “the potassiumcompound of the present invention”).

In general formula (1), A is a positive divalent element in groups 7 to11 of the periodic table. In terms of the ease of insertion andextraction of potassium ions, as well as capacity and potential, A ispreferably manganese, iron, cobalt, nickel, copper, or the like.

In general formula (1), B is positive tetravalent silicon, germanium,titanium, or manganese.

A and B may be the same elements. That is, both A and B may bemanganese. However, the potassium compound of the present inventionexcludes a case in which A is manganese and B is titanium, and a case inwhich A is cobalt and B is silicon.

In general formula (1), k is 0.6 to 1.5, and preferably 0.7 to 1.2 interms of the ease of insertion and extraction of potassium ions, as wellas capacity and potential. n is 1.5 to 2.5, and preferably 1.7 to 2.3 interms of the ease of insertion and extraction of potassium ions, as wellas capacity and potential. m is 3.5 to 4.5, and preferably 3.7 to 4.3 interms of the ease of insertion and extraction of potassium ions, as wellas capacity and potential.

Specific examples of the potassium compound of the present inventioninclude K_(n)Fe²⁺ _(k)Si⁴⁺O₄, K_(n)Fe²⁺ _(k)Ge⁴⁺O₄, K_(n)Fe²⁺_(k)Ti⁴⁺O₄, K_(n)Fe²⁺ _(k)Mn⁴⁺O₄, K_(n)Mn²⁺ _(k)Si⁴⁺O₄, K_(n)Mn²⁺_(k)Ge⁴⁺O₄, K_(n)Mn²⁺ _(k)Mn⁴⁺O₄, K_(n)Co²⁺ _(k)Ge⁴⁺O₄, K_(n)Co²_(k)Ti⁴⁺O₄, K_(n)Co²⁺ _(k)Mn⁴⁺O₄, K_(n)Ni²⁺ _(k)Si⁴⁺O₄, K_(n)Ni²⁺_(k)Ge⁴⁺O₄, K_(n)Ni²⁺ _(k)Ti⁴⁺O₄, K_(n)Ni²⁺ _(k)Mn⁴⁺O₄, K_(n)Cu²⁺_(k)Si⁴⁺O₄, K_(n)Cu²⁺ _(k)Ge⁴⁺O₄, K_(n)Cu²⁺ _(k)Ti⁴⁺O₄, K_(n)Cu²⁺_(k)Mn⁴⁺O₄, and the like. Of these, in terms of the ease of insertionand extraction of potassium ions, as well as capacity and potential, itis preferable to use K_(n)Fe²⁺ _(k)Si⁴⁺O₄, K_(n)Fe²⁺ _(k)Ge⁴⁺O₄,K_(n)Fe²⁺ _(k)Ti⁴⁺O₄, K_(n)Fe²⁺ _(k)Mn⁴⁺O₄, K_(n)Mn²⁺ _(k)Si⁴⁺O₄,K_(n)Mn²⁺ _(k)Ge⁴⁺O₄, K_(n)Mn²⁺ _(k)Mn⁴⁺O₄, K_(n)Co²⁺ _(k)Ge⁴⁺O₄,K_(n)Co²⁺ _(k)Ti⁴⁺O₄, K_(n)Ni²⁺ _(k)Si⁴⁺O₄, K_(n)Ni²⁺ _(k)Ge⁴⁺O₄,K_(n)Ni²⁺ _(k)Ti⁴⁺O₄, K_(n)Cu²⁺ _(k)Si⁴⁺O₄, K_(n)Cu²⁺ _(k)Ge⁴⁺O₄, etc.;and it is more preferable to use K_(n)Fe²⁺ _(k)Si⁴⁺O₄, K_(n)Fe²⁺_(k)Ge⁴⁺O₄, K_(n)Fe²⁺ _(k)Ti⁴⁺O₄, K_(n)Fe²⁺ _(k)Mn⁴⁺O₄, K_(n)Mn²⁺_(k)Si⁴⁺O₄, K_(n)Co²⁺ _(k)Ge⁴⁺O₄, K_(n)Co²⁺ _(k)Ti⁴⁺O₄, K_(n)Ni²⁺_(k)Si⁴⁺O₄, K_(n)Ni²⁺ _(k)Ge⁴⁺O₄, K_(n)Ni²⁺ _(k)Ti⁴⁺O₄, K_(n)Cu²⁺_(k)Si⁴⁺O₄, etc.

The potassium compound of the present invention can take any crystalstructure, such as a cubic structure, a tetragonal structure, anorthorhombic structure, or a monoclinic structure. In particular, thepotassium compound of the present invention preferably has a cubicstructure, a tetragonal structure, an orthorhombic structure, amonoclinic structure, etc., as a main phase. For example, K_(n)Mn²⁺_(k)Si⁴⁺O₄ and K_(n)Fe²⁺ _(k)Si⁴⁺O₄ preferably have a cubic structure asa main phase; K_(n)Co²⁺ _(k)Ge⁴⁺O₄ and K_(n)Ni²⁺ _(k)Si⁴⁺O₄ preferablyhave a tetragonal structure as a main phase; K_(n)Fe²⁺ _(k)Ge⁴⁺O₄,K_(n)Fe²⁺ _(k)Ti⁴⁺O₄, and K_(n)Fe²⁺ _(k)Mn⁴⁺O₄ preferably have anorthorhombic structure as a main phase; and K_(n)Co²⁺ _(k)Ti⁴⁺O₄ andK_(n)Cu²⁺ _(k)Si⁴⁺O₄ preferably have a monoclinic structure as a mainphase. The amount of the main phase crystal structure present in thepotassium compound of the present invention is not limited and ispreferably 80 mol % or more, and more preferably 90 mol % or more basedon the entire potassium compound of the present invention. Thus, thepotassium compound of the present invention can be formed of a materialhaving a single phase crystal structure, or a material having anothercrystal structure, as long as the effect of the present invention is notimpaired. The crystal structure of the potassium compound of the presentinvention is confirmed by X-ray diffraction measurement.

The potassium compound of the present invention has diffraction peaks atvarious positions in the X-ray diffractogram obtained using CuKaradiation. For example, K_(n)Fe²⁺ _(k)Si⁴⁺O₄, K_(n)Fe²⁺ _(k)Ge⁴⁺O₄,K_(n)Fe²⁺ _(k)Ti⁴⁺O₄, K_(n)Fe²⁺ _(k)Mn⁴⁺O₄, K_(n)Mn²⁺ _(k)Si⁴⁺O₄,K_(n)Co²⁺ _(k)Ge⁴⁺O₄, etc., have the strongest peak at a diffractionangle 2θ of 30.8 to 33.9°, and preferably further have peaks at least ata diffraction angle 2θ of 18.4 to 21.5°, 35.1 to 41.1°, 43.5 to 47.7°,48.2 to 52.8°, 55.2 to 58.8°, 63.0 to 71.7°, etc. Moreover, K_(n)Cu²⁺_(k)Si⁴⁺O₄ etc. have the strongest peak at a diffraction angle 2θ of34.7 to 36.6°, and preferably further have peaks at least at adiffraction angle 2θ of 24.6 to 27.7°, 28.8 to 30.6°, 31.2 to 33.6°,37.9 to 44.6°, 47.8 to 50.3°, 51.2 to 52.6°, 53.0 to 54.9°, 57.4 to59.1°, 60.9 to 62.9°, 65.0 to 69.4°, 71.6 to 73.3°, 74.5 to 76.8°, 79.5to 81.6°, 82.0 to 84.6°, etc. Furthermore, K_(n)Co²⁺ _(k)Ti⁴⁺O₄,K_(n)Ni²⁺ _(k)Si⁴⁺O₄, etc., have the strongest peak at a diffractionangle 2θ of 41.5 to 45.2°, and preferably further have peaks at least ata diffraction angle 2θ of 35.7 to 38.9°, 60.8 to 65.3°, 73.1 to 81.5°,etc. In the present specification, the strongest peak means the peakwith the highest intensity.

From the viewpoint of easy potassium ion insertion and extraction,capacity, and potential, the mean particle diameter of the potassiumcompound of the present invention having the crystal structure andcomposition mentioned above is preferably 0.2 to 200 μm, and morepreferably 0.5 to 150 μm. The mean particle diameter of the potassiumcompound of the present invention is measured by electron microscope(SEM) observation.

2. Method for Producing Potassium Compound

The potassium compound of the present invention can be obtained, forexample, by a production method comprising a heating step of heating amixture containing potassium, a positive divalent element in groups 7 to11 of the periodic table, positive tetravalent silicon, positivetetravalent germanium, positive tetravalent titanium or positivetetravalent manganese, and oxygen. The method is explained in detailbelow.

(1) Starting Material Compound

In the production method of the present invention, a mixture containingpotassium, an element in groups 7 to 11 of the periodic table, silicon,germanium, titanium or manganese, and oxygen is subjected to a heatingstep. The starting material compounds for obtaining the mixturecontaining potassium, an element in groups 7 to 11 of the periodictable, silicon, germanium, titanium or manganese, and oxygen may be amixture that in the end contains potassium, an element in groups 7 to 11of the periodic table, silicon, germanium, titanium or manganese, andoxygen at a specific ratio. Usable examples include potassium-containingcompounds, manganese-containing compounds, iron-containing compounds,cobalt-containing compounds, nickel-containing compounds,copper-containing compounds, silicon-containing compounds,germanium-containing compounds, titanium-containing compounds,oxygen-containing compounds, and the like.

The types of potassium-containing compounds, manganese-containingcompounds, iron-containing compounds, cobalt-containing compounds,nickel-containing compounds, copper-containing compounds,silicon-containing compounds, germanium-containing compounds,titanium-containing compounds, oxygen-containing compounds, etc., arenot limited. Four or more compounds each containing a respective elementamong potassium, manganese, iron, cobalt, nickel, copper, silicon,germanium, titanium, oxygen, and the like can be mixed for use.Alternatively, less than four compounds can be mixed using a compoundsimultaneously containing two or more elements among potassium,manganese, iron, cobalt, nickel, copper, silicon, germanium, titanium,oxygen, and the like as part of the starting materials.

These starting material compounds are preferably compounds that do notcontain metal elements (particularly rare metal elements) other thanpotassium, manganese, iron, cobalt, nickel, copper, silicon, germanium,titanium, oxygen, etc. Moreover, the elements other than potassium,manganese, iron, cobalt, nickel, copper, silicon, germanium, titanium,oxygen, etc., contained in the starting material compounds arepreferably those that undergo extraction and volatilization upon heattreatment in a non-oxidizing atmosphere, described later.

Specific examples of such starting material compounds are as follows.Examples of potassium-containing compounds include metal potassium (K),potassium hydroxide (KOH), potassium nitrate (KNO₃), potassium chloride(KCl), potassium carbonate (K₂CO₃), potassium azide (KN₃), potassiumoxalate (K₂C₂O₄), and the like. Examples of manganese-containingcompounds include metal manganese (Mn); manganese oxides, such asmanganese(II) oxide (MnO) and manganese(IV) oxide (MnO₂); manganesehydroxides, such as manganese(II) hydroxide (Mn(OH)₂) and manganese(IV)hydroxide (Mn(OH)₄); manganese(II) carbonate (MnCO₃); manganese(II)oxalate (MnC₂O₄); and the like. Examples of iron-containing compoundsinclude metal iron (Fe); iron oxides, such as iron(II) oxide (FeO) andiron(III) oxide (Fe₂O₃); iron hydroxides, such as iron(II) hydroxide(Fe(OH)₂) and iron(III) hydroxide (Fe(OH)₃); iron carbonates, such asiron(II) carbonate (FeCO₃) and iron(III) carbonate (Fe₂(CO₃)₃); iron(II)oxalate (FeC₂O₄); and the like. Examples of cobalt-containing compoundsinclude metal cobalt (Co), cobalt oxide (CoO), cobalt hydroxide (CoOH),cobalt carbonate (CoCO₃), cobalt oxalate (CoC₂O₄), etc. Examples ofnickel-containing compounds include metal nickel (Ni); nickel oxides,such as nickel(I) oxide (Ni₂O) and nickel(II) oxide (NiO); nickelhydroxides, such as nickel(I) hydroxide (NiOH) and nickel(II) hydroxide(Ni(OH)₂); nickel(II) carbonate (NiCO₃); nickel(II) oxalate (NiC₂O₄);and the like. Examples of copper-containing compounds include metalcopper (Cu), copper oxide (CuO), copper hydroxide (CuOH), coppercarbonate (CuCO₃), copper oxalate (CuC₂O₄), and the like. Examples ofsilicon-containing compounds include silicon (Si), silicon oxide (SiO₂),and the like. Examples of germanium-containing compounds includegermanium (Ge), germanium oxide (GeO₂), and the like. Examples oftitanium-containing compounds include metal titanium (Ti); titaniumoxide (TiO₂); titanium oxides, such as titanium hydroxide (Ti(OH)₄); andthe like. Examples of oxygen-containing compounds include potassiumhydroxide (KOH); potassium carbonate (K₂CO₃); manganese oxides, such asmanganese(II) oxide (MnO) and manganese(IV) oxide (MnO₂); manganesehydroxides, such as manganese(II) hydroxide (Mn(OH)₂) and manganese(IV)hydroxide (Mn(OH)₄); manganese(II) carbonate (MnCO₃); manganese(II)oxalate (MnC₂O₄); iron oxides, such as iron(II) oxide (FeO) andiron(III) oxide (Fe₂O₃); iron hydroxides, such as iron(II) hydroxide(Fe(OH)₂) and iron(III) hydroxide (Fe(OH)₃); iron carbonates, such asiron(II) carbonate (FeCO₃) and iron(III) carbonate (Fe₂(CO₃)₃); iron(II)oxalate (FeC₂C₄); cobalt oxide (CoO); cobalt hydroxide (CoOH); cobaltcarbonate (CoCO₃); cobalt oxalate (CoC₂O₄); nickel oxides, such asnickel(I) oxide (Ni₂O) and nickel(II) oxide (NiO); nickel hydroxides,such as nickel(I) hydroxide (NiOH) and nickel(II) hydroxide (Ni(OH)₂);nickel(II) carbonate (NiCO₃); nickel(II) oxalate (NiC₂O₄); copper oxide(CuO); copper hydroxide (CuOH); copper carbonate (CuCO₃); copper oxalate(CuC₂O₄); silicon oxide (SiO₂); germanium oxide (GeO₂); and the like.Moreover, hydrates of these starting material compounds can also beused.

In the present invention, the above starting material compounds may becommercial products, or may be separately synthesized.

The shape of these starting material compounds is not particularlylimited, and a powder shape is preferable in terms of handlingproperties. Moreover, in terms of reactivity, fine particles arepreferable, and a powder shape having a mean particle diameter of 1 μmor less (particularly about 60 to 80 nm) is preferable. The meanparticle diameter of the starting material compounds is measured byelectron microscope observation (SEM).

The mixture containing potassium, an element in groups 7 to 11 of theperiodic table, silicon, germanium, titanium or manganese, and oxygencan be obtained by mixing necessary materials among the startingmaterial compounds explained above.

The mixing ratio of the starting material compounds is not particularlylimited. It is preferable to mix the starting material compounds so asto obtain the composition of a potassium compound, which is the finalproduct. The mixing ratio of the starting material compounds ispreferably determined so that the ratio of each element contained in therespective starting material compounds is the same as the ratio of eachelement in the target composite oxide. Specifically, in terms of theease of insertion and extraction of potassium ions, as well as capacityand potential, the ratio of potassium: an element in groups 7 to 11 ofthe periodic table: silicon, germanium, titanium or manganese is, forexample, preferably 30 to 70 mol %:15 to 35 mol %:15 to 35 mol %, andmore preferably 40 to 60 mol %:20 to 30 mol %:20 to 30 mol %.

(2) Production Method

The mixing method for producing a mixture containing potassium, anelement in groups 7 to 11 of the periodic table, silicon, germanium,titanium or manganese, and oxygen is not particularly limited. A methodthat can uniformly mix each starting material compound can be used.Usable examples include mortar mixing, mechanical milling,coprecipitation, a method in which each component is dispersed in asolvent and then mixed, and a method in which the components are mixedby dispersing them in a solvent at once. Of these methods, when mortarmixing is used, the potassium compound of the present invention can beobtained more simply. To obtain a more uniform mixture, coprecipitationcan be used.

When mechanical milling is performed as the mixing method, examples ofusable mechanical milling devices include a ball mill, a vibration mill,a turbo mill, a disc mill, and the like; and preferably a ball mill. Inthis case, it is preferable to perform mixing and heat treatmentsimultaneously.

The atmosphere during mixing and heating is not particularly limited.For example, an inert gas atmosphere, such as argon or nitrogen, can beused. Further, mixing and heating may be performed under reducedpressure (e.g., under vacuum).

In the heat treatment of the mixture containing potassium, an element ingroups 7 to 11 of the periodic table, silicon, germanium, titanium ormanganese, and oxygen, the heating temperature is preferably 600 to1500° C., more preferably 650 to 1300° C., and even more preferably 700to 1000° C., because operation can be more easily performed, and thecrystallinity and electrode characteristics (capacity and potential) ofthe resulting potassium compound can be further improved. The heatingtime is not particularly limited. For example, the heating time ispreferably 10 minutes to 48 hours, and more preferably 30 minutes to 24hours.

3. Potassium Ion Secondary Battery Positive Electrode Active

Material

Because of the above composition, crystal structure, etc., the potassiumcompound of the present invention enables insertion and extraction ofpotassium ions, and is thus useful as a potassium ion secondary batterypositive electrode active material.

In addition to the potassium compound of the present invention, thefollowing potassium compound also enables insertion and extraction ofpotassium ions, and is thus useful as a potassium ion secondary batterypositive electrode active material. Specifically, this potassiumcompound is represented by general formula (2): K_(n)A_(k)BO_(m) whereinA is a positive divalent element in groups 7 to 11 of the periodictable; B is positive tetravalent silicon, germanium, titanium ormanganese; k is 0.6 to 1.5; n is 1.5 to 2.5; and m is 3.5 to 4.5.

In general formula (2), A and B can be those mentioned in generalformula (1). General formula (2) includes a case in which A is manganeseand B is titanium, and a case in which A is cobalt and B is silicon.

In general formula (2), k is 0.6 to 1.5, and preferably 0.7 to 1.2 interms of the ease of insertion and extraction of potassium ions, as wellas capacity and potential. n is 1.5 to 2.5, and preferably 1.7 to 2.3 interms of the ease of insertion and extraction of potassium ions, as wellas capacity and potential. m is 3.5 to 4.5, and preferably 3.7 to 4.3 interms of the ease of insertion and extraction of potassium ions, as wellas capacity and potential.

Specific examples of the potassium compound represented by generalformula (1) or (2), which can be thus used for the potassium ionsecondary battery positive electrode active material of the presentinvention, include K_(n)Fe²⁺ _(k)Si⁴⁺O₄, K_(n)Fe²⁺ _(k)Ge⁴⁺O₄, K_(n)Fe²⁺_(k)Ti⁴⁺O₄, K_(n)Fe²⁺ _(k)Mn⁴⁺O₄, K_(n)Mn²⁺ _(k)Si⁴⁺O₄, K_(n)Mn²⁺_(k)Ge⁴⁺O₄, K_(n)Mn²⁺ _(k)Ti⁴⁺O₄, K_(n)Mn²⁺ _(k)Mn⁴⁺O₄, K_(n)Co²⁺_(k)Si⁴⁺O₄, K_(n)Co²⁺ _(k)Ge⁴⁺O₄, K_(n)Co²⁺ _(k)Ti⁴⁺O₄, K_(n)Co²⁺_(k)Mn⁴⁺O₄, K_(n)Ni²⁺ _(k)Si⁴⁺O₄, K_(n)Ni²⁺ _(k)Ge⁴⁺O₄, K_(n)Ni²⁺_(k)Ti⁴⁺O₄, K_(n)Ni²⁺ _(k)Mn⁴⁺O₄, K_(n)Cu²⁺ _(k)Si⁴⁺O₄, K_(n)Cu²⁺_(k)Ge⁴⁺O₄, K_(n)Cu²⁺ _(k)Ti⁴⁺O₄, K_(n)Cu²⁺ _(k)Mn⁴⁺O₄, and the like. Ofthese, in terms of the ease of insertion and extraction of potassiumions, as well as capacity and potential, it is preferable to useK_(n)Fe²⁺ _(k)Si⁴⁺O₄, K_(n)Fe²⁺ _(k)Ge⁴⁺O₄, K_(n)Fe²⁺ _(k)Ti⁴⁺O₄,K_(n)Fe²⁺ _(k)Mn⁴⁺O₄, K_(n)Mn²⁺ _(k)Si⁴⁺O₄, K_(n)Mn²⁺ _(k)Ge⁴⁺O₄,K_(n)Mn²⁺ _(k)Ti⁴⁺O₄, K_(n)Mn²⁺ _(k)Mn⁴⁺O₄, K_(n)Co²⁺ _(k)Si⁴⁺O₄,K_(n)Co²⁺ _(k)Ge⁴⁺O₄, K_(n)Co²⁺ _(k)Ti⁴⁺O₄, K_(n)Ni²⁺ _(k)Si⁴⁺O₄,K_(n)Ni²⁺ _(k)Ge⁴⁺O₄, K_(n)Ni²⁺ _(k)Ti⁴⁺O₄, K_(n)Cu²⁺ _(k)Si⁴⁺O₄,K_(n)Cu²⁺ _(k)Ge⁴⁺O₄, etc.; it is more preferable to use K_(n)Fe²⁺_(k)Si⁴⁺O₄, K_(n)Fe²⁺ _(k)Ge⁴⁺O₄, K_(n)Fe²⁺ _(k)Ti⁴⁺O₄, K_(n)Fe²⁺_(k)Mn⁴⁺O₄, K_(n)Mn²⁺ _(k)Si⁴⁺O₄, K_(n)Co²⁺ _(k)Si⁴⁺O₄, K_(n)Co²⁺_(k)Ge⁴⁺O₄, K_(n)Co²⁺ _(k)Ti⁴⁺O₄, K_(n)Ni²⁺ _(k)Si⁴⁺O₄, K_(n)Ni²⁺_(k)Ge⁴⁺O₄, K_(n)Ni²⁺ _(k)Ti⁴⁺O₄, K_(n)Cu²⁺ _(k)Si⁴⁺O₄, etc.; and it iseven more preferable to use K_(n)Co²⁺ _(k)Si⁴⁺O₄.

The crystal structure of the potassium compound represented by generalformula (1) or (2), which can be used for the potassium ion secondarybattery positive electrode active material of the present invention, isnot particularly limited. The potassium compound of the presentinvention can have a cubic structure, a tetragonal structure, anorthorhombic structure, a monoclinic structure, or the like. Inparticular, the potassium compound of the present invention preferablyhas a cubic structure, a tetragonal structure, an orthorhombicstructure, a monoclinic structure, or the like as a main phase. Forexample, K_(n)Mn²⁺ _(k)Si⁴⁺O₄ and K_(n)Fe²⁺ _(k)Si⁴⁺O₄ preferably have acubic structure as a main phase; K_(n)Co²⁺ _(k)Ge⁴⁺O₄ and K_(n)Ni²⁺_(k)Si⁴⁺O₄ preferably have a tetragonal structure as a main phase;K_(n)Fe²⁺ _(k)Ge⁴⁺O₄, K_(n)Fe²⁺ _(k)Ti⁴⁺O₄, K_(n)Fe²⁺ _(k)Mn⁴⁺O₄, andK_(n)Co²⁺ _(k)Si⁴⁺O₄ preferably have an orthorhombic structure as a mainphase; and K_(n)Co²⁺ _(k)Ti⁴⁺O₄ and K_(n)Cu²⁺ _(k)Si⁴⁺O₄ preferably havea monoclinic structure as a main phase. In the potassium compoundrepresented by general formula (1) or (2), which can be used for thepotassium secondary battery positive electrode active material of thepresent invention, the abundance of the crystal structure as a mainphase is not particularly limited, and is preferably 80 mol % or more,and more preferably 90 mol % or more, based on the entire potassiumcompound represented by general formula (1) or (2), which can be usedfor the potassium secondary battery positive electrode active materialof the present invention. Therefore, the potassium compound representedby general formula (1) or (2), which can be used for the potassium ionsecondary battery positive electrode active material of the presentinvention, can be a material having a single-phase crystal structure, ora material having another crystal structure, as long as the effect ofthe present invention is not impaired. The crystal structure of thepotassium compound represented by general formula (1) or (2), which canbe used for the potassium ion secondary battery positive electrodeactive material of the present invention, is confirmed by X-raydiffraction measurement.

Moreover, the potassium compound represented by general formula (1) or(2), which can be used for the potassium ion secondary battery positiveelectrode active material of the present invention, can have diffractionpeaks in various positions in an X-ray diffractogram obtained using CuKaradiation. For example, K_(n)Fe²⁺ _(k)Si⁴⁺O₄, K_(n)Fe²⁺ _(k)Ge⁴⁺O₄,K_(n)Fe²⁺ _(k)Ti⁴⁺O₄, K_(n)Fe²⁺ _(k)Mn⁴⁺O₄, K_(n)Mn²⁺ _(k)Si⁴⁺O₄,K_(n)Co²⁺ _(k)Ge⁴⁺O₄, K_(n)Co²⁺ _(k)Si⁴⁺O₄, etc., have the strongestpeak at a diffraction angle 2θ of 30.8 to 33.9°, and preferably furtherhave peaks at least at a diffraction angle 2θ of 18.4 to 21.5°, 35.1 to41.1°, 43.5 to 47.7°, 48.2 to 52.8°, 55.2 to 58.8°, 63.0 to 71.7°, etc.Moreover, K_(n)Cu²⁺ _(k)Si⁴⁺O₄ etc. have the strongest peak at adiffraction angle 2θ of 34.7 to 36.6°, and preferably further have peaksat least at a diffraction angle 20 of 24.6 to 27.7°, 28.8 to 30.6°, 31.2to 33.6°, 37.9 to 44.6°, 47.8 to 50.3°, 51.2 to 52.6°, 53.0 to 54.9°,57.4 to 59.1°, 60.9 to 62.9°, 65.0 to 69.4°, 71.6 to 73.3°, 74.5 to76.8°, 79.5 to 81.6°, 82.0 to 84.6°, etc. Furthermore, K_(n)Co²⁺_(k)Ti⁴⁺O₄, K_(n)Ni²⁺ _(k)Si⁴⁺O₄, etc., have the strongest peak at adiffraction angle 2θ of 41.5 to 45.2°, and preferably further have peaksat least at a diffraction angle 2θ of 35.7 to 38.9°, 60.8 to 65.3°, 73.1to 81.5°, etc.

The mean particle diameter of the potassium compound represented bygeneral formula (1) or (2) having the above crystal structure andcomposition, which can be used for the potassium ion secondary batterypositive electrode active material of the present invention, ispreferably 0.2 to 200 μm, and more preferably 0.5 to 150 μm, in terms ofthe ease of insertion and extraction of potassium ions, as well ascapacity and potential. The mean particle diameter of the potassiumcompound represented by general formula (1) or (2), which can be usedfor the potassium secondary battery positive electrode active materialof the present invention, is measured by electron microscope (SEM)observation.

In the potassium ion secondary battery positive electrode activematerial of the present invention, the potassium compound and a carbonmaterial (e.g., carbon black such as acetylene black) may form acomposite. The carbon material thereby suppresses the grain growthduring firing, which enables obtaining a fine particle potassium ionsecondary battery positive electrode active material having excellentelectrode properties. In this case, the content of the carbon materialin the potassium ion secondary battery positive electrode activematerial of the present invention is preferably adjusted to 3 to 20 mass%, and particularly preferably 5 to 15 mass %.

The potassium ion secondary battery positive electrode active materialof the present invention comprises the potassium compound mentionedabove. The potassium ion secondary battery positive electrode activematerial of the present invention can consist of the potassium compoundof the present invention, and can contain inevitable impurities inaddition to the potassium compound of the present invention. Examples ofsuch inevitable impurities include the starting material compoundsexplained above. The inevitable impurity can be contained in an amountof about 10 mol % or less, preferably about 5 mol % or less, and evenmore preferably about 2 mol % or less as long as the effect of thepresent invention is not impaired.

4. Potassium Ion Secondary Battery Positive Electrode and Potassium IonSecondary Battery

In the potassium ion secondary battery positive electrode and thepotassium ion secondary battery according to the present invention, thepotassium compound represented by general formula (1) or (2) is used asa positive active material, and other basic structures can be formed byreferring to known nonaqueous electrolyte lithium ion secondary batterypositive electrodes and nonaqueous electrolyte lithium ion secondarybatteries. For example, a positive electrode, negative electrode, andseparator can be arranged in a battery container in such a manner thatthe positive electrode is isolated from the negative electrode by theseparator. Subsequently, the battery container is filled with anonaqueous electrolyte solution, and then sealed, thus producing thepotassium ion secondary battery of the present invention. The potassiumion secondary battery used in the present invention may be a potassiumsecondary battery. In this specification, “potassium ion secondarybattery” means a secondary battery in which potassium ions are carrierions, and “potassium secondary battery” means a secondary battery inwhich potassium metal or potassium alloy is used as a negative electrodeactive material.

The positive electrode can take a structure in which a positiveelectrode material containing the potassium compound represented bygeneral formula (1) or (2) is supported on a positive electrode currentcollector. For example, the positive electrode can be produced byapplying a positive electrode mixture containing the potassium compoundrepresented by general formula (1) or (2), a conductive material, andoptionally a binder to a positive electrode current collector.

Examples of conductive materials include acetylene black, Ketjenblack,carbon nanotube, vapor-grown carbon fibers, carbon nanofibers, graphite,corks, and like carbon materials. The shape of the conductive materialis not limited, and powders, for example, can be used.

Examples of binders include fluororesins, such as polyvinylidenefluoride resin and polytetrafluoroethylene.

The contents of components in the positive electrode material are notlimited and can be suitably determined. For example, it is preferablethat the potassium compound represented by general formula (1) or (2) iscontained in an amount of 50 to 95 vol % (particularly, 70 to 90 vol %),the conductive material is contained in an amount of 2.5 to 25 vol %(particularly, 5 to 15 vol %), and the binder is contained in an amountof 2.5 to 25 vol % (particularly 5 to 15 vol %).

Examples of materials composing the positive electrode current collectorinclude aluminum, platinum, molybdenum, stainless steel, etc. Examplesof the shape of the positive electrode current collector include aporous body, foil, plate, mesh formed of fiber, etc.

It is preferable that the application amount of the positive electrodematerial relative to the positive electrode current collector issuitably determined in accordance with the use etc. of the potassium ionsecondary battery.

Examples of negative electrode active materials composing the negativeelectrode include potassium metal; silicon; silicon-containing Clathratecompounds; potassium alloy; ternary or quaternary oxides represented byM¹M² ₂O₄ (M¹: Co, Ni, Mn, Sn, etc. M²: Mn, Fe, Zn, etc.); metal oxidesrepresented by M³304 (M³: Fe, Co, Ni, Mn, etc.), M⁴ ₂O₃ (M⁴: Fe, Co, Ni,Mn, etc.), M_(n)V₂O₆, M⁴O₂ (M⁴: Sn, Ti, etc.), M²O (M²: Fe, Co, Ni, Mn,Sn, Cu, etc.), etc.; graphite, hard carbon, soft carbon, graphene;carbon materials mentioned above; Lepidocrocite-typeK_(0.8)Li_(0.2)Ti_(1.67)O₄; KC₈; KTi₃O₄; K₂Ti₆O₁₃; K₂Ti_(n)O_(2n+1)(n=3, 4, 6, 8); K₂SiP₂; KSi₂P₃; MnSnO₃; K_(1.4)Ti₈O₁₆;K_(1.5)Ti_(6.5)V_(1.5)O₁₆; K_(1.4)Ti_(6.6)Mn_(1.4)O₁₆; Zn₃ (HCOO)₆; Co₃(HCOO)₆; Zn_(1.5)Co_(1.5) (HCOO)₆; KVMoO₆; AV₂O₆ (A=Mn, Co, and Ni, Cu);Mn₂GeO₄; Ti₂ (SO₄)₃; KTi₂ (PO₄)₃; SnO₂; Nb₂O₅; TiO₂; Te; VOMoO₄; TiS₂;TaS₂; MoSe₂; SnSe₂; SnS₅; SnO₂; Sb₂O₃; NiCo₂S₄; Sb₂O₄; Ni₃S₂; FeS₂;Nb₂O₅; K_(0.3)MoO₂; K₂Ti₃O₇; K₂Ti₂O₅; Fe₃O₄; Fe₂O₃; Co₃O₄; CuO; Sb; Ge;P; TiO₂; KTiO₂; SnSb; organic-based compounds, such as polyacethylene(PAc), polyanthracene, polyparaphenylene (PPP), 1,4-benzenedicarboxylate(BDC), polyaniline (Pan), polypyrrole (PPy), polythiophene(PTh), tetraethylthiuram disulfide (TETD),poly(2,5-dimercapto-1,3,4-thiadiazole) (PDMcT),poly(2,2′-dithiodianyline) (PDTDA), poly(5,8-dihydro-1H,4H-2,3,6,7-tetrathia-anthracene) (PDTTA),poly(2,2,6,6-tetramethylpiperidine-1-oxyl-4-yl methacrylate) (PTMA),K₂C₆H₄O₄, K₂C₁₀H₂O₄, K₂C₅O₅.2H₂O, K₄C₈H₂O₆, K₂C₆H₄O₄, K₂C₁₀H₂O₄,K₂C₁₄H₆O₄, K₄C₆O₆, K₄C₂₄H₈O₈, K₄C₆O₆, K₂C₆O₆, K₂C₆H₂O₄, K₂C₁₄H₆O₄,K₂C₈H₄O₄, K₂C₁₄H₄N₂O₄, K₂C₆H₄O₄, K₂C₁₈H₁₂ ^(O) ₈, K₂C₁₆H₈O₄, andK₂C₁₀H₂N₂O₄; etc. Examples of potassium alloys include alloys containingpotassium and aluminum as constituent elements, alloys containingpotassium and zinc as constituent elements, alloys containing potassiumand manganese as constituent elements, alloys containing potassium andbismuth as constituent components, alloys containing potassium andnickel as constituent elements, alloys containing potassium and antimonyas constituent elements, alloys containing potassium and tin asconstituent elements, and alloys containing potassium and indium asconstituent elements; quaternary layered carbon or nitrogen compounds,such as MXene-based alloys comprising metal (scandium, titanium,vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum,etc.) and carbon as constituent elements, M⁵ _(x)BC₃-based alloys (M⁵:scandium, titanium, vanadium, chromium, zirconium, niobium, molybdenum,hafnium, tantalum, etc.), etc.; alloys containing potassium and lead asconstituent elements; etc.

The negative electrode can be formed of a negative electrode activematerial or can take a structure in which a negative electrode materialcontaining a negative electrode active material, a conductive material,and optionally a binder is supported on a negative electrode currentcollector. When the structure in which a negative electrode material issupported on a negative electrode current collector is taken, a negativeelectrode mixture containing a negative electrode active material, aconductive material, and optionally a binder is applied to a negativeelectrode current collector, thus producing a negative electrode.

When a negative electrode is formed of a negative electrode activematerial, the negative electrode active material mentioned above can beformed into a shape (e.g., plate) that is suitable for the electrode.

When the structure in which a negative electrode material is supportedon a negative electrode current collector is used, the types of theconductive material and binder, and the contents of the negativeelectrode active material, conductive material, and binder, are the sameas those mentioned for the positive electrode. Examples of materialscomposing the negative electrode current collector include aluminum,copper, nickel, stainless steel, etc. Of these, since potassium does notform an alloy with aluminum, a low-cost aluminum negative electrodecurrent collector can be used rather than an expensive copper negativeelectrode current collector. Examples of the shape of the negativeelectrode current collector include a porous body, foil, plate, meshformed of fiber, etc. It is preferable that the application amount ofthe negative electrode material relative to the negative electrodecurrent collector is suitably determined in accordance with the use etc.of the potassium ion secondary battery.

Any separator can be used as long as the separator is made of a materialcapable of isolating the positive electrode and the negative electrodein a battery, and retaining an electrolyte solution to ensure ionconductivity between the positive electrode and the negative electrode.Examples of separators include materials in the form of a porous film,non-woven fabric, and woven fabric that are made of polyolefin resin,such as polyethylene, polypropylene, polyimide, polyvinyl alcohol, andterminated amino polyethylene oxide; fluororesin, such aspolytetrafluoroethylene; acrylic resin; nylon; aromatic aramid;inorganic glass; ceramics; etc.

Nonaqueous electrolyte solutions preferably contain potassium ions.Examples of such electrolyte solutions include potassium salt solutions,ion liquids formed of a potassium-containing inorganic material, etc.

Examples of potassium salts include potassium inorganic salt compounds,such as potassium halides (e.g., potassium chloride, potassium bromide,and potassium iodide), potassium perchlorate, potassiumtetrafluoroborate, potassium hexafluorophosphorate, and potassiumhexafluoroarsenate; potassium organic salt compounds, such as potassiumbis(trifluoromethylsulfonyl) imide, potassiumbis(perfluoroethanesulfony)imide, potassium benzoate, potassiumsalicylate, potassium phthalate, potassium acetate, potassiumpropionate, and Grignard reagent; etc.

Examples of solvents include carbonate compounds, such as propylenecarbonate, ethylene carbonate, dimethyl carbonate, ethylmethylcarbonate, and diethyl carbonate; lactone compounds such asγ-butyrolactone and γ-valerolactone; ether compounds, such astetrahydrofuran, 2-methyltetrahydrofuran, diethylether, diisopropylether, dibutyl ether, methoxy methane, N,N-dimethylformamide, glyme,N-propyl-N-methyl pyrrolidinium bis(trifluoromethane sulfonyl) imide,dimethoxyethane, dimethoxymethane, diethoxymethane, diethoxyethane, andpropyleneglycol dimethyl ether; acetonitrile; etc.

A solid electrolyte can also be used in place of the nonaqueouselectrolyte solution. Examples of solid electrolytes include potassiumion conductors, such as KH₂PO₄, KZr₂(PO₄)₃, K₉Fe(MoO₄)₆, K₄Fe₃(PO₄)₂P₂₇,and K₃MnTi(PO₄)₃.

Because of the use of the potassium compound of the present invention,the potassium ion secondary battery of the present invention ensureshigher potential and energy density in an oxidation reduction reaction(charge-discharge reaction), and moreover, it is highly safe (polyanionskeleton) and useful. Accordingly, the potassium ion secondary batteryof the present invention is, for example, suitably used in devices thatare desired to have a smaller size and higher performance.

EXAMPLES

The present invention is explained in detail below with reference toExamples and Comparative Examples. The present invention is, needless tosay, not limited to these.

The following reagents were used in the Examples.

-   K₂CO₃ (produced by Rare Metallic Co., Ltd., 99.9% (3N))-   FeO (produced by Wako Pure Chemical Industries, Ltd., 99.5%)-   FeC₂O₄.2H₂O (produced by Junsei Chemical Co., Ltd., 99.9% (3N))-   SiO₂ (produced by Kanto Chemical Co., Inc., 99.9% (3N), sedimentary    (amorphous))-   GeO₂ (produced by Kanto Chemical Co., Inc. 99.99% (4N))-   TiO₂(A) (produced by Rare Metallic, 99.99% (4N))-   MnO₂ (produced by Rare Metallic, 99.99% (4N))-   MnO (produced by Kojundo Chemical Laboratory Co., Ltd., 99.9% (3N))-   MnC₂O₄ (produced by Kojundo Chemical Laboratory Co., Ltd., 99.9%    (3N))-   CoC₂O₄ (produced by Kojundo Chemical Laboratory Co., Ltd., 99% (2N))-   CoO (produced by Rare Metallic, 99.9% (3N))-   NiO (produced by Wako Pure Chemical Industries, Ltd., 99.9% (3N))-   Ni(OH)₂ (produced by Kojundo Chemical Laboratory Co., Ltd., 99.9%    (3N))-   CuO (produced by Kojundo Chemical Laboratory Co., Ltd., 99.99% (4N))    Measurement of Powder X-Ray Diffraction (XRD)

X-ray diffraction measurements were used for the identification ofsamples obtained by synthesis and the collection of structure analysisdata. The X-ray diffraction measurement device used was RINT2200(produced by Rigaku). The X radiation source used was CuKαmonochromatized by a monochromator. Data were collected undermeasurement conditions in which the tube voltage was 50 kV and the tubecurrent was 300 mA. The scanning rate was set so that the intensity wasabout 10000 counts. Samples used for measurement were sufficientlyground so that uniform particles were obtained. Rietveld analysis wasconducted for structure analysis, and JANA-2006 was used as theanalyzing program.

Example 1 K_(n)Fe²⁺ _(k)Si⁴⁺O₄ Example 1-1

K₂CO₃, FeO, and SiO₂ were used as starting material powders. Operationwas performed in a dry room in order to prevent water absorption ofK₂CO₃.

K₂CO₃, FeO, and SiO₂ were weighed so that the molar ratio of potassium,iron, and silicon was 2:1:1, and mixed in an agate mortar for about 30minutes. The mixture was formed into pellets, and fired in an electricfurnace in argon at 800° C. for 1 hour. As a sample preparation methodfor avoiding the influence of air exposure due to the hygroscopicity ofthe product, the product obtained after firing was placed in a glove boxin which an Ar atmosphere was maintained, and stored in an environmentwithout contact with air. The product (K₂Fe²⁺Si⁴⁺O₄) was confirmed byX-ray diffraction.

Example 1-2

K₂CO₃, SiO₂, and FeC₂O₄.2H₂O as starting material powders were weighedso that the molar ratio of potassium, iron, and silicon was 2:1:1, andthey were placed in a chromium steel container together with 10 zirconiaballs (diameter: 15 mm). Acetone was added, and grinding and mixing wereperformed with a planetary ball mill (Fritsch; P-6) at 600 rpm for 6hours. After the acetone was removed under reduced pressure, thecollected powder was formed into pellets at 40 MPa, and fired in an Arflow at 700° C., 800° C., or 850° C. for 2 hours. The heating rate inthis case was set to 400° C./h. The cooling rate was 100° C./h until300° C., followed by natural cooling to room temperature. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The X-ray diffractionpatterns confirmed that the obtained product (K₂Fe²⁺Si⁴⁺O₄) was the sameas the product obtained in Example 1-1. FIG. 1 shows the results.

Example 1-3

When a target substance is mixed with a carbon material that functionsas a conductive material, and the resulting mixture is fired, the targetsubstance and the carbon material spontaneously form a uniformcomposite, and the carbon material suppresses the grain growth duringfiring; thus, it is possible to synthesize fine particles havingexcellent electrode characteristics. K₂CO₃, SiO₂, and FeC₂O₄.2H₂O wereweighed so that the molar ratio of potassium, iron, and silicon was2:1:1. Further, acetylene black was added as a conductive material sothat 10% mass ratio of carbon remained in the final product, and themixture was placed in a chromium steel container together with 10zirconia balls (diameter: 15 mm). Acetone was added, and grinding andmixing were performed with a planetary ball mill (Fritsch; P-6) at 400rpm for 24 hours. After the acetone was removed under reduced pressure,the collected powder was formed into pellets by hand pressing, and firedin an Ar flow at 800° C. for 2 hours. The heating rate in this case wasset to 400° C./h until 800° C. The cooling rate was 100° C./h until 300°C., followed by natural cooling to room temperature. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The X-ray diffractionpatterns confirmed that the obtained product (K₂Fe²⁺Si⁴⁺O₄) was the sameas the products obtained in Examples 1-1 and 1-2. FIG. 2 shows theresults.

According to the results shown in FIGS. 1 and 2, when the firingtemperature was 650° C. or more, multiple main peaks were observed atleast at a 2θ value of 19 to 35°. Since these peaks correspond tosingle-phase K₂Fe²⁺Si⁴⁺O₄, it is found that single-phase K₂Fe²⁺Si⁴⁺O₄ isobtained as a product. Moreover, the peaks observed at a 2θ value of 30to 35° are higher at a higher firing temperature; thus, it is found thata higher firing temperature is preferable.

FIG. 3 shows the results of comparing K₂Fe²⁺Si⁴⁺O₄ obtained in Example1-2 with other iron silicate compounds (Na₂Fe²⁺Si⁴⁺O₄ andLi₂Fe²⁺Si⁴⁺O₄). Na₂Fe²⁺Si⁴⁺O₄ and Li₂Fe²⁺Si⁴⁺O₄ are samples synthesizedas in Example 1-2, except that starting material compounds were changed,and firing was performed at 800° C. for 2 hours. As a result, the X-raydiffraction pattern of K₂Fe²⁺Si⁴⁺O₄ is clearly different from those ofthe other iron silicate compounds (Na₂Fe²⁺Si⁴⁺O₄ and Li₂Fe²⁺Si⁴⁺O₄);thus, it can be understood that a new crystal phase is formed.

Moreover, FIGS. 4 and 7 show the X-ray diffraction patterns ofK₂Fe²⁺Si⁴⁺O₄ obtained in Example 1-2 determined by powder X-raydiffraction. The results reveal that the crystals of the obtainedK₂Fe²⁺Si⁴⁺O₄ have the strongest peak at a diffraction angle, representedby 2θ, of 31.5 to 33.0°, and further have peaks at a diffraction angleof 19.0 to 20.3°, 39.3 to 40.4°, 45.5 to 47.2°, 48.2 to 49.0°, 50.4 to51.6°, 53.3 to 53.8°, 57.0 to 58.4°, 57.9 to 58.8°, 61.0 to 62.1°, 66.7to 68.7°, and 76.4 to 77.9°. These results reveal that the crystals ofthe obtained K₂Fe²⁺Si⁴⁺O₄ have a cubic structure (space group Fd-3 m),that the lattice constants are a=b=c=7.8297(6)Å and α=β=γ=90°, and thatthe unit lattice volume (V) is 480.01(3)Å³. The reliability factors wereas follows: R_(wp)=4.09%, R_(p)=3.01%, χ²=1.24. FIGS. 8 to 10 show theresults of confirming the obtained product (K₂Fe²⁺Si⁴⁺O₄) in more detailusing Rietveld X-ray diffraction patterns. The results confirmed thatthe obtained K₂Fe²⁺Si⁴⁺O₄ had an orthorhombic structure (space groupPca2₁ S.G), and that the lattice constants were as follows:a=11.1023(19)Å, b=5.5338(4)Å, c=15.7469(29)Å, and V=967.4(2)Å³. Thus, itcan be understood that the obtained sample has an orthorhombicstructure.

Further, K₂Fe²⁺Si⁴⁺O₄ obtained in Example 1-2 was observed using ascanning electron microscope. FIG. 5 shows the results. In FIG. 5, thescale bar represents 0.5 μm. The results shown in FIG. 5 reveal thatK₂Fe²⁺Si⁴⁺O₄ having a particle diameter of around 20 μm was obtained.Further, K₂Fe²⁺Si⁴⁺O₄ obtained in Example 1-2 was observed using ascanning electron microscope and a high-resolution scanning electronmicroscope. FIGS. 11 to 13 show the results. The results shown in FIGS.11 to 13 reveal that K₂Fe²⁺Si⁴⁺O₄ having a particle diameter of around 5μm was obtained.

Moreover, the thermal stability measurement of K₂Fe²⁺Si⁴⁺O₄ obtained inExample 1-2 was performed using TG-DTA. FIG. 6 shows the results. As aresult, the obtained K₂Fe²⁺Si⁴⁺O₄ was stable in a wide temperature range(a temperature range up to 500° C.). A battery having high-level thermalstability can be constructed using this material as a battery material.

Further, K₂Fe²⁺Si⁴⁺O₄ obtained in Example 1-2 was gradually heated, andthe thermal stability and color tone were observed. FIG. 14 shows theresults. As a result, it can be understood that the thermal stability ofK₂Fe²⁺Si⁴⁺O₄ was superior to that of K₂Co²⁺Si⁴⁺O₄.

Example 2 K_(n)Fe²⁺ _(k)Ge⁴⁺O₄ Example 2-1

K₂CO₃, FeC₂O₄.2H₂O, and GeO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, FeC₂O₄.2H₂O, and GeO₂ (produced by Kanto Chemical Co., Inc.,99.99% (4 N)) were weighed so that the molar ratio of potassium, iron,and germanium was 2:1:1, and mixed in an agate mortar for about 30minutes. The mixture was formed into pellets, and fired in an electricfurnace in argon at 800° C. for 1 hour. As a sample preparation methodfor avoiding the influence of air exposure due to the hygroscopicity ofthe product, the product obtained after firing was placed in a glove boxin which an Ar atmosphere was maintained, and stored in an environmentwithout contact with air. The product (K₂Fe²⁺Ge⁴⁺O₄) was confirmed byX-ray diffraction.

Example 2-2

K₂CO₃, FeO, and GeO₂ were used as starting material powders. Operationwas performed in a dry room in order to prevent the water absorption ofK₂CO₃.

K₂CO₃, FeO, and GeO₂ were weighed so that the molar ratio of potassium,iron, and germanium was 2:1:1, and mixed in an agate mortar for about 30minutes. The mixture was formed into pellets, and fired in an electricfurnace in argon at 800° C. for 1 hour. As a sample preparation methodfor avoiding the influence of air exposure due to the hygroscopicity ofthe product, the product obtained after firing was placed in a glove boxin which an Ar atmosphere was maintained, and stored in an environmentwithout contact with air. The X-ray diffraction pattern confirmed thatthe obtained product (K₂Fe²⁺Ge⁴⁺O₄) was the same as the product obtainedin Example 2-1.

FIG. 7 shows the X-ray diffraction pattern of K₂Fe²⁺Ge⁴⁺O₄ obtained inExample 2-1. The results of FIG. 7 reveal that the crystals of theobtained K₂Fe²⁺Ge⁴⁺O₄ have the strongest peak at a diffraction angle,represented by 2θ, of 31.2 to 33.9° in the X-ray diffraction patterndetermined by powder X-ray diffraction, and further have peaks at adiffraction angle of 18.5 to 20.1°, 36.2 to 40.2°, 40.8 to 43.6°, 45.1to 46.7°, 49.5 to 50.9°, 56.0 to 58.2°, 59.9 to 61.7°, 65.9 to 68.3°,69.8 to 71.2°, and 75.0 to 77.4°. These results reveal that the crystalsof the obtained K₂Fe²⁺Ge⁴⁺O₄ have an orthorhombic structure, that thelattice constants are a=3.951 Å, b=7.167 Å, c=11.918 Å, and α=β=γ=90°,and that the unit lattice volume (V) is 337.5 Å³.

The product (K₂Fe²⁺Ge⁴⁺O₄) obtained in Example 2-1 was confirmed byX-ray diffraction patterns. FIG. 10 shows the results. The resultsconfirmed that the same results as FIG. 7 were obtained.

K₂Fe²⁺Ge⁴⁺O₄ obtained in Example 2-1 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 15 and 16 show the results. The results shown in FIGS. 15 and 16reveal that K₂Fe²⁺Ge⁴⁺O₄ having a particle diameter of around 7 μm wasobtained.

Example 3 K_(n)Fe²⁺ _(k)Ti⁴⁺O₄ Example 3-1

K₂CO₃, FeC₂O₄.2H₂O, and TiO₂ (A) were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, FeC₂O₄.2H₂O, and TiO₂ (A) were weighed so that the molar ratio ofpotassium, iron, and titanium was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Fe²⁺Ti⁴⁺O₄) was confirmed by X-ray diffraction.

FIG. 7 shows the X-ray diffraction pattern of K₂Fe²⁺Ti⁴⁺O₄ obtained inExample 3-1. The results of FIG. 7 reveal that the crystals of theobtained K₂Fe²⁺Ti⁴⁺O₄ have the strongest peak at a diffraction angle,represented by 2θ, of 31.0 to 33.0° in the X-ray diffraction patterndetermined by powder X-ray diffraction, and further have peaks at adiffraction angle of 18.5 to 20.2°, 28.3 to 30.0°, 34.0 to 35.6°, 38.7to 40.0°, 40.5 to 42.7°, 44.7 to 46.1°, 49.0 to 50.6°, 55.7 to 57.4°,59.5 to 60.9°, 65.2 to 67.2°, 68.9 to 70.4°, and 74.2 to 76.2°. Theseresults reveal that the crystals of the obtained K₂Fe²⁺Ti⁴⁺O₄ have anorthorhombic structure (space group Fddd), that the lattice constantsare a=6.979 Å, b=7.989 Å, c=11.918 Å, and α=β=γ=90°, and that the unitlattice volume (V) is 337.5 Å³.

Example 3-2

K₂CO₃, FeO, and TiO₂(A) were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, FeO, and TiO₂(A) were weighed so that the molar ratio ofpotassium, iron, and titanium was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The X-ray diffractionpattern confirmed that the obtained product (K₂Fe²⁺Ti⁴⁺O₄) was the sameas the product obtained in Example 3-1.

The product (K₂Fe²⁺Ti⁴⁺O₄) obtained in Example 3-1 was confirmed byX-ray diffraction pattern. FIG. 10 shows the results. As a result, itwas confirmed that the same results as FIG. 7 were obtained.

K₂Fe²⁺Ti⁴⁺O₄ obtained in Example 3-1 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 17 and 18 show the results. The results shown in FIGS. 17 and 18reveal that K₂Fe²⁺Ti⁴⁺O₄ having a particle diameter of around 7 μm wasobtained.

Example 4 K_(n)Fe²⁺ _(k)Mn⁴⁺O₄ Example 4-1

K₂CO₃, FeC₂O₄.2H₂O, and MnO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, FeC₂O₄.2H₂O, and MnO₂ were weighed so that the molar ratio ofpotassium, iron, and manganese was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Fe²⁺Mn⁴⁺O₄) was confirmed by X-ray diffraction.

FIG. 7 shows the X-ray diffraction pattern of K₂Fe²⁺Mn⁴⁺O₄ obtained inExample 4-1. The results of FIG. 7 reveal that the crystals of theobtained K₂Fe²⁺Mn⁴⁺O₄ have the strongest peak at a diffraction angle,represented by 2θ, of 30.8 to 33.9° in the X-ray diffraction patterndetermined by powder X-ray diffraction, and further have peaks at adiffraction angle of 18.4 to 21.5°, 24.8 to 27.1°, 35.1 to 37.2°, 37.6to 39.5°, 43.5 to 46.6°, 55.2 to 58.3°, 63.0 to 67.4°, 68.7 to 72.2°,and 74.5 to 77.4°. These results reveal that the crystals of theobtained K₂Fe²⁺Mn⁴⁺O₄ have an orthorhombic structure, that the latticeconstants are a=4.806 Å, b=4.609 Å, c=6.945 Å, and α=β=γ=90°, and thatthe unit lattice volume (V) is 153.8 Å³.

Example 4-2

K₂CO₃, FeO, and MnO₂ were used as starting material powders. Operationwas performed in a dry room in order to prevent the water absorption ofK₂CO₃.

K₂CO₃, FeO, and MnO₂ were weighed so that the molar ratio of potassium,iron, and manganese was 2:1:1, and mixed in an agate mortar for about 30minutes. The mixture was formed into pellets, and fired in an electricfurnace in argon at 800° C. for 1 hour. As a sample preparation methodfor avoiding the influence of air exposure due to the hygroscopicity ofthe product, the product obtained after firing was placed in a glove boxin which an Ar atmosphere was maintained, and stored in an environmentwithout contact with air. The X-ray diffraction pattern confirmed thatthe obtained product (K₂Fe²⁺Mn⁴⁺O₄) was the same as the product obtainedin Example 4-1.

Example 4-3

K₂CO₃, FeC₂O₄.2H₂O, and MnO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, FeC₂O₄.2H₂O, and MnO₂ were weighed so that the molar ratio ofpotassium, iron, and manganese was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800 to 1100° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Fe²⁺Mn⁴⁺O₄) was confirmed by X-ray diffraction. FIG. 19 shows theresults. The results reveal that the same crystals as those of Examples4-1 and 4-2 were obtained at any temperature.

The product (K₂Fe²⁺Mn⁴⁺O₄) obtained by firing at 800° C. or 1000° C. wasconfirmed by X-ray diffraction patterns. FIGS. 10, 20, and 21 show theresults. The results confirmed that the obtained K₂Fe²⁺Mn⁴⁺O₄ was thesame crystals as those of Examples 4-1 and 4-2.

K₂Fe²⁺Mn⁴⁺O₄ obtained in Example 4-3 was observed using a scanningelectron microscope. FIG. 22 shows the results. The results shown inFIG. 22 reveal that K₂Fe²⁺Mn⁴⁺O₄ having a particle diameter of around 3μm was obtained.

K₂Fe²⁺Mn⁴⁺O₄ obtained in Example 4-1 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 23 and 24 show the results. The results shown in FIGS. 23 and 24reveal that K₂Fe²⁺Mn⁴⁺O₄ having a particle diameter of around 3 μm wasobtained.

Example 5 K_(n)Co²⁺ _(k)Si⁴⁺O₄ Example 5-1

K₂CO₃, CoC₂O₄, and SiO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, CoC₂O₄, and SiO₂ were weighed so that the molar ratio ofpotassium, cobalt, and silicon was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Co²⁺Si⁴⁺O₄) was confirmed by X-ray diffraction. FIG. 25 shows theresults.

FIGS. 9, 25, and 28 show the X-ray diffraction patterns of K₂Co²⁺Si⁴⁺O₄obtained in Example 5-1. The results shown in FIGS. 25 and 28 revealthat the crystals of the obtained K₂Co²⁺Si⁴⁺O₄ have the strongest peakat a diffraction angle, represented by 2θ, of 32.1 to 33.6° in the X-raydiffraction patterns determined by powder X-ray diffraction, and furtherhave peaks at a diffraction angle of 19.4 to 20.6°, 39.7 to 41.1°, 46.0to 47.7°, 50.9 to 52.4°, 57.8 to 58.3°, 67.9 to 69.5°, and 77.2 to79.0°. These results reveal that the crystals of the obtained K₂CoSiO₄have an orthorhombic structure (space group Fd-3m), that the latticeconstants are a=b=c=7.735 Å and α=β=γ=90°, and that the unit latticevolume (V) is 462.8 Å³.

Further, K₂Co²⁺Si⁴⁺O₄ obtained in Example 5-1 was observed using ascanning electron microscope. FIG. 26 shows the results. In FIG. 26, thescale bar represents 2.5 μm. The results shown in FIG. 26 reveal thatK₂CoSiO₄ having a particle diameter of around 1.0 μm was obtained.

Moreover, the thermal stability measurement of K₂Co²⁺Si⁴⁺O₄ obtained inExample 5-1 was performed using TG-DTA. FIG. 27 shows the results. As aresult, the obtained K₂Co²⁺Si⁴⁺O₄ was stable in a wide temperature range(a temperature range up to 500° C.). A battery having high-level thermalstability can be constructed using this material as a battery material.

Example 5-2

K₂CO₃, CoO, and SiO₂ were used as starting material powders. Operationwas performed in a dry room in order to prevent the water absorption ofK₂CO₃.

K₂CO₃, CoO, and SiO₂ were weighed so that the molar ratio of potassium,cobalt, and silicon was 2:1:1, and mixed in an agate mortar for about 30minutes. The mixture was formed into pellets, and fired in an electricfurnace in argon at 800° C. for 1 hour. As a sample preparation methodfor avoiding the influence of air exposure due to the hygroscopicity ofthe product, the product obtained after firing was placed in a glove boxin which an Ar atmosphere was maintained, and stored in an environmentwithout contact with air. The X-ray diffraction patterns confirmed thatthe obtained product (K₂Co²⁺Si⁴⁺O₄) was the same as the product obtainedin Example 5-1. FIG. 25 shows the results.

K₂Co²⁺Si⁴⁺O₄ obtained in Example 5-1 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 29 and 30 show the results. The results shown in FIGS. 29 and 30reveal that K₂Co²⁺Si⁴⁺O₄ having a particle diameter of around 3 μm wasobtained.

Further, K₂Co²⁺Si⁴⁺O₄ obtained in Example 5-1 was gradually heated, andthe thermal stability and color tone were observed. FIG. 14 shows theresults. As a result, the thermal stability of K₂Co²⁺Si⁴⁺O₄ was notexcellent as those of K₂Mn²⁺Si⁴⁺O₄ and K₂Fe²⁺Si⁴⁺O₄.

Example 6 K_(n)Co²⁺ _(k)Ge⁴⁺O₄ Example 6-1

K₂CO₃, CoC₂O₄, and GeO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, CoC₂O₄, and GeO₂ were weighed so that the molar ratio ofpotassium, cobalt, and germanium was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Co²⁺Ge⁴⁺O₄) was confirmed by X-ray diffraction.

FIG. 28 shows the X-ray diffraction pattern of K₂Co²⁺Ge⁴⁺O₄ obtained inExample 6-1. The results shown in FIG. 28 reveal that the crystals ofthe obtained K₂Co²⁺Ge⁴⁺O₄ have the strongest peak at a diffractionangle, represented by 2θ, of 32.2 to 33.9° in the X-ray diffractionpattern determined by powder X-ray diffraction, and further have peaksat a diffraction angle of 18.8 to 20.6°, 30.6 to 32.0°, 36.3 to 38.9°,44.2 to 45.8°, 50.3 to 52.8°, 55.5 to 57.8°, 58.4 to 60.7°, and 67.5 to71.7°. These results reveal that the crystals of the obtainedK₂Co²⁺Ge⁴⁺O₄ have a tetragonal structure, that the lattice constants area=b=5.712 Å, c=7.434 Å, and α=β=γ=90°, and that the unit lattice volume(V) is 242.5 Å³.

Example 6-2

K₂CO₃, CoO, and GeO₂ were used as starting material powders. Operationwas performed in a dry room in order to prevent the water absorption ofK₂CO₃.

K₂CO₃, CoO, and GeO₂ were weighed so that the molar ratio of potassium,cobalt, and germanium was 2:1:1, and mixed in an agate mortar for about30 minutes. The mixture was formed into pellets, and fired in anelectric furnace in argon at 800° C. for 1 hour. As a sample preparationmethod for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The X-ray diffractionpatterns confirmed that the obtained product (K₂Co²⁺Ge⁴⁺O₄) was the sameas the product obtained in Example 6-1.

K₂Co²⁺Ge⁴⁺O₄ obtained in Example 6-1 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 31 and 32 show the results. The results shown in FIGS. 31 and 32reveal that K₂Co²⁺Ge⁴⁺O₄ having a particle diameter of around 5 μm wasobtained.

Example 7 K_(n)Co²⁺ _(k)Ti⁴⁺O₄ Example 7-1

K₂CO₃, CoC₂O₄, and TiO₂ (A) were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, CoC₂O₄, and TiO₂(A) were weighed so that the molar ratio ofpotassium, cobalt, and titanium was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Co²⁺Ti⁴⁺O₄) was confirmed by X-ray diffraction.

FIG. 28 shows the X-ray diffraction pattern of K₂Co²⁺Ti⁴⁺O₄ obtained inExample 7-1. The results shown in FIG. 28 reveal that the crystals ofthe obtained K₂Co²⁺Ti⁴⁺O₄ have the strongest peak at a diffractionangle, represented by 2θ, of 41.5 to 43.7° in the X-ray diffractionpattern determined by powder X-ray diffraction, and further have peaksat a diffraction angle of 28.1 to 30.7°, 31.1 to 32.6°, 34.7 to 35.6°,35.9 to 37.6°, 60.8 to 63.3°, 73.1 to 74.7°, and 76.8 to 78.8°. Theseresults reveal that the crystals of the obtained K₂Co²⁺Ti⁴⁺O₄ have amonoclinic structure, that the lattice constants are a=5.047 Å, b=5.659Å, c=6.269 Å, and β=100.44°, and that the unit lattice volume (V) is176.1 Å³.

Example 7-2

K₂CO₃, CoO, and TiO₂(A) were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, CoO, and TiO₂(A) were weighed so that the molar ratio ofpotassium, cobalt, and titanium was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The X-ray diffractionpattern confirmed that the obtained product (K₂Co²⁺Ti⁴⁺O₄) was the sameas the product obtained in Example 7-1.

K₂Co²⁺Ti⁴⁺O₄ obtained in Example 7-1 was observed using a scanningelectron microscope. FIG. 33 shows the results. The results shown inFIG. 33 reveal that K₂Co²⁺Ti⁴⁺O₄ having a particle diameter of around 20μm was obtained.

Example 8 K_(n)Ni²⁺ _(k)Si⁴⁺O₄ Example 8-1

K₂CO₃, NiO, and SiO₂ were used as starting material powders. Operationwas performed in a dry room in order to prevent the water absorption ofK₂CO₃.

K₂CO₃, NiO, and SiO₂ were weighed so that the molar ratio of potassium,nickel, and silicon was 2:1:1, and mixed in an agate mortar for about 30minutes. The mixture was formed into pellets, and fired in an electricfurnace in argon at 800° C. for 1 hour. As a sample preparation methodfor avoiding the influence of air exposure due to the hygroscopicity ofthe product, the product obtained after firing was placed in a glove boxin which an Ar atmosphere was maintained, and stored in an environmentwithout contact with air. The product (K₂Ni²⁺Si⁴⁺O₄) was confirmed byX-ray diffraction.

K₂Ni²⁺Si⁴⁺O₄ obtained in Example 8-1 was observed using a scanningelectron microscope. FIG. 34 shows the results. In FIG. 34, the scalebar represents 71.4 μm. The results shown in FIG. 34 reveal thatK₂Ni²⁺Si⁴⁺O₄ having a particle diameter of around 100 μm was obtained.

Example 8-2

K₂CO₃, Ni(OH)₂, and SiO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, Ni(OH)₂, and SiO₂ were weighed so that the molar ratio ofpotassium, nickel, and silicon was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800° C. and 900° C. for 1 hour. As asample preparation method for avoiding the influence of air exposure dueto the hygroscopicity of the product, the product obtained after firingwas placed in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The X-ray diffractionpatterns confirmed that the obtained product (K₂Ni²⁺Si⁴⁺O₄) was the sameas the product obtained in Example 8-1.

FIGS. 9 and 35 show the X-ray diffraction patterns of K₂Ni²⁺Si⁴⁺O₄obtained in Example 8-2. The obtained K₂Ni²⁺Si⁴⁺O₄ showed multiple mainpeaks at least at a 20 value of 25 to 65°. Since these peaks correspondto single-phase K₂Ni²⁺Si⁴⁺O₄, it is found that single-phase K₂Ni²⁺Si⁴⁺O₄is obtained as a product. It is also revealed that the crystals of theobtained K₂Ni²⁺Si⁴⁺O₄ have the strongest peak at a diffraction angle,represented by 2θ, of 44.0 to 45.0° in the X-ray diffraction patternsdetermined by powder X-ray diffraction, and further have peaks at adiffraction angle of 25.38 to 26.63°, 31.83 to 33.50°, 51.48 to 52.55°,and 75.96 to 77.17°. These results reveal that the crystals of theobtained K₂Ni²⁺Si⁴⁺O₄ have a tetragonal structure (space group I4₁/a cd), that the lattice constants are a=b=5.576 Å, c=3.534 Å, andα=β=γ=90°, and that the unit lattice volume (V) is 109.9 Å³.

Moreover, the thermal stability measurement of K₂Ni²⁺Si⁴⁺O₄ obtained inExample 8-2 was performed using TG-DTA. FIG. 36 shows the results. As aresult, the obtained K₂Ni₂₊Si⁴⁺O₄ was stable in a wide temperature range(a temperature range up to 500° C.). A battery having high-level thermalstability can be constructed using this material as a battery material.

K₂Ni²⁺Si⁴⁺O₄ obtained in Example 8-2 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 37 and 38 show the results. The results shown in FIGS. 37 and 38reveal that K₂Ni²⁺Si⁴⁺O₄ having a particle diameter of around 10 μm wasobtained.

Example 9 K_(n)Cu²⁺ _(k)Si⁴⁺O₄ Example 9-1

K₂CO₃, CuO, and SiO₂ were used as starting material powders. Operationwas performed in a dry room in order to prevent the water absorption ofK₂CO₃.

K₂CO₃, CuO, and SiO₂ were weighed so that the molar ratio of potassium,copper, and silicon was 2:1:1, and mixed in an agate mortar for about 30minutes. The mixture was formed into pellets, and fired in an electricfurnace in argon at 800° C. for 1 hour. As a sample preparation methodfor avoiding the influence of air exposure due to the hygroscopicity ofthe product, the product obtained after firing was placed in a glove boxin which an Ar atmosphere was maintained, and stored in an environmentwithout contact with air. The product (K₂Cu²⁺Si⁴⁺O₄) was confirmed byX-ray diffraction.

Example 9-2

Treatment was performed as in Example 9-1, except that firing wasconducted in an air atmosphere, not an argon atmosphere. The X-raydiffraction pattern confirmed that the obtained product (K₂Cu²⁺Si⁴⁺O₄)was the same as the product obtained in Example 9-1.

FIG. 39 shows the X-ray diffraction patterns of K₂Cu²⁺Si⁴⁺O₄ obtained inExamples 9-1 and 9-2. The results shown in FIG. 39 reveal that thecrystals of the obtained K₂Cu²⁺Si⁴⁺O₄ have the strongest peak at adiffraction angle, represented by 2θ, of 34.7 to 36.6° in the X-raydiffraction patterns determined by powder X-ray diffraction, and furtherhave peaks at a diffraction angle of 24.6 to 27.7°, 28.8 to 30.6°, 31.2to 33.6°, 37.9 to 44.6°, 47.8 to 50.3°, 51.2 to 52.6°, 53.0 to 54.9°,57.4 to 59.1°, 60.9 to 62.9°, 65.0 to 69.4°, 71.6 to 73.3°, 74.5 to76.8°, 79.5 to 81.6°, and 82.0 to 84.6°. These results reveal that thecrystals of the obtained K₂Cu²⁺Si⁴⁺O₄ have a monoclinic structure, thatthe lattice constants are a=4.634 Å, b=3.420 Å, c=6.321 Å, and β=90.34°,and the unit lattice volume (V) is 100.2 Å³.

Example 10 K_(n)Mn²⁺ _(k)Si⁴⁺O₄ Example 10-1

K₂CO₃, MnC₂O₄, and SiO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, MnC₂O₄, and SiO₂ were weighed so that the molar ratio ofpotassium, manganese, and silicon was 2:1:1, and mixed in an agatemortar for about 30 minutes. The mixture was formed into pellets, andfired in an electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the obtained product, the product obtained afterfiring was placed in a glove box in which an Ar atmosphere wasmaintained, and stored in an environment without contact with air. Theproduct (K₂Mn²⁺Si⁴⁺O₄) was confirmed by X-ray diffraction.

Example 10-2

K₂CO₃, MnO, and SiO₂ were used as starting material powders. Operationwas performed in a dry room in order to prevent the water absorption ofK₂CO₃.

K₂CO₃, MnO, and SiO₂ were weighed so that the molar ratio of potassium,manganese, and silicon was 2:1:1, and mixed in an agate mortar for about30 minutes. The mixture was formed into pellets, and fired in anelectric furnace in a nitrogen gas atmosphere at 700° C., 800° C., or900° C. for 1 hour. As a sample preparation method for avoiding theinfluence of air exposure due to the hygroscopicity of the obtainedproduct, the product obtained after firing was placed in a glove box inwhich an Ar atmosphere was maintained, and stored in an environmentwithout contact with air. The X-ray diffraction patterns confirmed thatthe obtained product (K₂Mn²⁺Si⁴⁺O₄) was the same as the product obtainedin Example 10-1.

FIG. 40 shows the X-ray diffraction patterns of K₂Mn²⁺Si⁴⁺O₄ obtained inExample 10-2. The results shown in FIG. 40 reveal that when the firingtemperature is 650° C. or more, multiple main peaks are observed atleast at a 20 value of 19 to 68°. Since these peaks correspond tosingle-phase K₂Mn²⁺Si⁴⁺O₄, the results shown in FIG. 40 reveal thatsingle-phase K₂Mn²⁺Si⁴⁺O₄ is obtained as a product. Moreover, the peaksobserved at a 20 value of 30 to 35° are higher at a higher firingtemperature; thus, it is found that the firing temperature is 650° C. ormore, and that a higher temperature is preferable. It is also revealedthat the crystals of the obtained K₂Mn²⁺Si⁴⁺O₄ have the strongest peakat a diffraction angle, represented by 2θ, of 31.7 to 33.1° in the X-raydiffraction patterns determined by powder X-ray diffraction, and furtherhave peaks at a diffraction angle of 19.1 to 20.4°, 39.3 to 40.6°, 45.8to 47.1°, 50.4 to 51.5°, 56.9 to 58.7°, 66.8 to 68.5°, 69.7 to 70.9°,and 76.2 to 77.9°. These results reveal that the crystals of theobtained K₂MnSiO₄ have a cubic structure (space group Fd-3 m), that thelattice constants are a=b=c=7.826 Å and α=β=γ=90°, and that the unitlattice volume (V) is 479.3 Å³.

Further, K₂Mn²⁺Si⁴⁺O₄ obtained in Example 10-1 was observed using ascanning electron microscope. FIG. 41 shows the results. In FIG. 41, thescale bar represents 3.0 μm. The results shown in FIG. 41 reveal thatK₂Mn²⁺Si⁴⁺O₄ having a particle diameter of around 1.0 μm was obtained.

Moreover, the thermal stability measurement of K₂Mn²⁺Si⁴⁺O₄ obtained inExample 10-1 was performed using TG-DTA. FIG. 42 shows the results. As aresult, the obtained K₂Mn²⁺Si⁴⁺O₄ was stable in a wide temperature range(a temperature range up to 500° C.). A battery having high-level thermalstability can be constructed using this material as a battery material.

The product (K₂Mn²⁺Si⁴⁺O₄) obtained in Example 10-1 was confirmed byRietveld X-ray diffraction patterns. FIGS. 9 and 43 show the results.The results confirmed that the obtained K₂Mn²⁺Si⁴⁺O₄ had an orthorhombicstructure (space group Pca2₁ S.G), and that the lattice constants werea=11.1306(3)Å, b=5.5334(1)Å, c=15.7817(29)Å, and V=972.1(4)Å³.

Moreover, K₂Mn²⁺Si⁴⁺O₄ obtained in Example 10-1 was observed using ascanning electron microscope and a high-resolution scanning electronmicroscope. FIGS. 44 to 46 show the results. The results shown in FIGS.44 to 46 reveal that K₂Mn²⁺Si⁴⁺O₄ having a particle diameter of around 2μm was obtained.

Further, K₂Mn²⁺Si⁴⁺O₄ obtained in Example 10-1 was gradually heated, andthe thermal stability and color tone were observed. FIG. 14 shows theresults. As a result, it can be understood that the thermal stability ofK₂Mn²⁺Si⁴⁺O₄ was superior to that of K₂Co²⁺Si⁴⁺O₄.

Example 11 K_(n)Mn²⁺ _(k)Ti⁴⁺O₄ Example 11-1

K₂CO₃, MnC₂O₄, and TiO₂ (A) were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, MnC₂O₄, and TiO₂ (A) were weighed so that the molar ratio ofpotassium, manganese, and titanium was 2:1:1, and mixed in an agatemortar for about 30 minutes. The mixture was formed into pellets, andfired in an electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Mn²⁺Ti⁴⁺O₄) was confirmed by X-ray diffraction.

Example 11-2

K₂CO₃, MnO, and TiO₂(A) were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, MnO, and TiO₂(A) were weighed so that the molar ratio ofpotassium, manganese, and titanium was 2:1:1, and mixed in an agatemortar for about 30 minutes. The mixture was formed into pellets, andfired in an electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The X-ray diffractionpattern confirmed that the obtained product (K₂Mn²⁺Ti⁴⁺O₄) was the sameas the product obtained in Example 11-1.

Example 12 K_(n)Mn²⁺ _(k)Ge⁴⁺O₄ Example 12-1

K₂CO₃, MnC₂O₄, and GeO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, MnC₂O₄, and GeO₂ were weighed so that the molar ratio ofpotassium, manganese, and germanium was 2:1:1, and mixed in an agatemortar for about 30 minutes. The mixture was formed into pellets, andfired in an electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Mn²⁺Ge⁴⁺O₄) was confirmed by X-ray diffraction.

Example 12-2

K₂CO₃, MnO, and GeO₂ were used as starting material powders. Operationwas performed in a dry room in order to prevent the water absorption ofK₂CO₃.

K₂CO₃, MnC₂O₄, and GeO₂ were weighed so that the molar ratio ofpotassium, manganese, and germanium was 2:1:1, and mixed in an agatemortar for about 30 minutes. The mixture was formed into pellets, andfired in an electric furnace in argon at 800° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The X-ray diffractionpattern confirmed that the obtained product (K₂Mn²⁺Ge⁴⁺O₄) was the sameas the product obtained in Example 12-1.

K₂Mn²⁺Ge⁴⁺O₄ obtained in Example 12-1 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 47 and 48 show the results. The results shown in FIGS. 47 and 48reveal that K₂Mn²⁺Ge⁴⁺O₄ having a particle diameter of around 10 μm wasobtained.

The elemental analysis of K₂Mn²⁺Ge⁴⁺O₄ obtained in Example 12-1 wasperformed by SEM-EDX. FIG. 49 shows the results. The results reveal thatthe amount of K is 47.63 mass %, the amount of Mn is 29.67 mass %, andthe amount of Ge is 22.70 mass %, based on 100 mass % of the total metalamount.

Example 13 K_(n)Mn²⁺ _(k)Mn⁴⁺O₄

K₂CO₃, MnO₂, and MnC₂O₄ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, MnO₂, and MnC₂O₄ were weighed so that the molar ratio ofpotassium, manganese (IV), and manganese (II) was 2:1:1, and they wereplaced in a chromium steel container together with 10 zirconia balls(diameter: 15 mm). Acetone was added, and grinding and mixing wereperformed with a planetary ball mill (Fritsch; P-6) at 600 rpm for 6hours. After the acetone was removed under reduced pressure, thecollected powder was formed into pellets at 40 MPa, and fired in an Arflow at 700° C., 800° C., or 850° C. for 2 hours. The heating rate inthis case was set to 400° C./h. The cooling rate was 100° C./h until300° C., followed by natural cooling to room temperature. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Mn²⁺Mn⁴⁺O₄) was confirmed by X-ray diffraction.

K₂Mn²⁺Mn⁴⁺O₄ obtained in Example 13 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 50 and 51 show the results. The results shown in FIGS. 50 and 51reveal that K₂Mn²⁺Mn⁴⁺O₄ having a particle diameter of around 7 μm wasobtained.

The product (K₂Mn²⁺Mn⁴⁺O₄) obtained by firing at 800° C. or 1000° C. wasconfirmed by Rietveld X-ray diffraction patterns. FIGS. 20, 21, and 52show the results. The results confirmed that the obtained K₂Mn²⁺Mn⁴⁺O₄had a monoclinic structure (space group C2/m S.G), and that the latticeconstants were a=14.0040(20)Å, b=2.9564(2)Å, c=10.7458(8)Å, β=96.63(0)°,and V=441.9(1)Å³.

The elemental analysis of K₂Mn²⁺Mn⁴⁺O₄ obtained in Example 13 wasperformed by SEM-EDX. FIG. 53 shows the results. The results reveal thatthe amount of K is 49.18 mass %, and the amount of Mn is 50.82 mass %,based on 100 mass % of the total metal amount.

K₂Mn²⁺Mn⁴⁺O₄ obtained in Example 13 was evaluated by HAADF-STEM. FIG. 54shows the results. As a result, it can be understood that all of theelements K, Mn, and O are uniformly present throughout the product.

Example 14 K_(n)Ni²⁺ _(k)Mn⁴⁺O₄ Example 14

K₂CO₃, Ni(OH)₂, and MnO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, Ni(OH)₂, and MnO₂ were weighed so that the molar ratio ofpotassium, nickel, and manganese was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800 to 1000° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Ni²⁺Mn⁴⁺O₄) was confirmed by X-ray diffraction.

The product (K₂Mn²⁺Mn⁴⁺O₄) obtained by firing at 800° C. or 1000° C. wasconfirmed by X-ray diffraction patterns. FIGS. 20 and 21 show theresults.

K₂Ni²⁺Mn⁴⁺O₄ obtained in Example 14 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 55 and 56 show the results. The results shown in FIGS. 55 and 56reveal that K₂Ni²⁺Mn⁴⁺O₄ having a particle diameter of around 7 μm wasobtained.

The elemental analysis of K₂Ni²⁺Mn⁴⁺O₄ obtained in Example 14 wasperformed by SEM-EDX. FIG. 57 shows the results. The results reveal thatthe amount of K is 51.40 mass %, the amount of Mn is 26.38 mass %, andthe amount of Ni is 22.22 mass %, based on 100 mass % of the total metalamount.

Example 15 K_(n)Co²⁺ _(k)Mn⁴⁺O₄

K₂CO₃, CoC₂O₄, and MnO₂ were used as starting material powders.Operation was performed in a dry room in order to prevent the waterabsorption of K₂CO₃.

K₂CO₃, CoC₂O₄, and MnO₂ were weighed so that the molar ratio ofpotassium, cobalt, and manganese was 2:1:1, and mixed in an agate mortarfor about 30 minutes. The mixture was formed into pellets, and fired inan electric furnace in argon at 800 to 1000° C. for 1 hour. As a samplepreparation method for avoiding the influence of air exposure due to thehygroscopicity of the product, the product obtained after firing wasplaced in a glove box in which an Ar atmosphere was maintained, andstored in an environment without contact with air. The product(K₂Co²⁺Mn⁴⁺O₄) was confirmed by X-ray diffraction.

The product (K₂Mn²⁺Mn⁴⁺O₄) obtained by firing at 800° C. or 1000° C. wasconfirmed by X-ray diffraction patterns. FIGS. 20, 21, and 58 show theresults. The results confirmed that the obtained K₂Co²⁺Mn⁴⁺O₄ had amonoclinic structure (space group C2/m S.G), and that the latticeconstants were a=12.9719(24)Å, b=2.8233(5)Å, c=10.4685(9)Å, β=95.22(1)°,and V=381.8(1)Å³.

K₂Co²⁺Mn⁴⁺O₄ obtained in Example 15 was observed using a scanningelectron microscope and a high-resolution scanning electron microscope.FIGS. 59 and 60 show the results. The results shown in FIGS. 59 and 60reveal that K₂Co²⁺Mn⁴⁺O₄ having a particle diameter of around 6 μm wasobtained.

The elemental analysis of K₂Co²⁺Mn⁴⁺O₄ obtained in Example 15 wasperformed by SEM-EDX. FIG. 61 shows the results. The results reveal thatthe amount of K is 46.48 mass %, the amount of Mn is 27.88 mass %, andthe amount of Co is 25.64 mass %, based on 100 mass % of the total metalamount.

Table 1 below collectively shows the lattice parameters of the materialsobtained in the above Examples. In Table 1, β and V represent the angleand volume of lattice constant. The error of the lattice constantparameters a, b, and c is within 0.1 Å, and the error of β is within15°.

Moreover, in Table 1 below, “capacity” refers to theoreticalcharge-discharge capacity during insertion and extraction of potassium,and is determined in the following manner.

The amount of 2 potassium ions extracted from K₂A²⁺B⁴⁺O₄ is representedby z, and the insertion reaction proceeds as shown in the followingformula.K₂A²⁺B⁴⁺O₄↔A⁽²⁺²⁾⁺B⁴⁺O₄+2K⁺+2e⁻

This reaction is performed at a constant current I (A). When t (sec)represents the time of current flow, m (g) represents the weight ofK₂A²⁺B⁴⁺O₄, M represents molecular weight, and F represents the Faradayconstant, the capacity and the amount of reacted lithium z arerepresented as follows:

$\mspace{20mu}{{z \times \frac{m}{M}} = { \frac{it}{F}\Rightarrow z  = \frac{itF}{Fm}}}$$\mspace{20mu}{{{Capacity}\text{/}{{mAh} \cdot g^{- 1}}} = {{\frac{it}{m} \times \frac{1000}{3600}} = \frac{it}{3.6\; m}}}$$\mspace{20mu}{{Specifically},{{{Capacity}\lbrack {mAhg}^{- 1} \rbrack} = {\frac{n \times {F\lbrack {C\;{mol}^{- 1}} \rbrack}}{M_{w}\lbrack {gmol}^{- 1} \rbrack} = {\frac{n \times {96485\lbrack C\rbrack}}{M_{w}\lbrack g\rbrack} = {\frac{n \times {96485\lbrack{As}\rbrack}}{M_{w}} = {\frac{n \times 96485 \times {\frac{1000}{3600}\lbrack{mAh}\rbrack}}{M_{w}\lbrack g\rbrack} = {\frac{26801 \times n}{M_{w}}\lbrack {mAhg}^{- 1} \rbrack}}}}}}}$$\mspace{20mu}{{ {K_{2}{CoSiO}_{4}}rightarrow{{CoSiO}_{4} + {2e^{-}} + {2{K^{+}( {{\therefore n} = 2} )}}} \mspace{20mu}\because{Capacity}} = {\frac{26801 \times {n( {= 2} )}}{M_{w}( {= 229.21} )} \cong {234\lbrack {mAhg}^{- 1} \rbrack}}}$

The theoretical capacity can be calculated from these formulas. In thecase of K₂Co²⁺Si⁴⁺O₄, up to 2 potassium ions can be theoreticallyextracted; thus, the theoretical capacity is determined to be 233.9mAh·g⁻¹.

TABLE 1 compound symmetry lattice a (Å) b (Å) c (Å) β (°) V (Å³)capacity (mAhg⁻¹) K₂FeSiO₄ Ed-3m cubic 7.839 7.829 7.829 90.00 479.9 237K₂FeGeO₄ orthorhombic 3.951 7.167 11.918 90.00 337.5 198 K₂FeTiO₄ Fdddorthorhombic 6.979 7.989 11.918 90.00 337.5 218 K₂FeMnO₄ orthorhombic4.806 4.609 6.945 90.00 153.8 212 K₂MnSiO₄ Fd-3m cubic 7.826 7.826 7.82690.00 479.3 238 K₂NiSiO₄ I41/a c d tetragonal 5.576 5.576 3.534 90.00109.9 234 K₂CoSiO₄ Fd-3m cubic 7.735 7.735 7.735 90.00 462.8 234K₂CuSiO₄ monoclinic 4.634 3.420 6.321 90.34 100.2 229 K₂CuMnO₄monoclinic 4.658 3.169 11.579 106.39 163.9 206 K₂MnGeO₄ orthorhombic11.131 5.533 15.782 90.00 972.1 199 K₂CoTiO₄ monoclinic 5.047 5.6596.269 100.44 176.1 215 K₂CoGeO₄ tetragonal 5.712 5.712 7.434 90.00 242.5196 K₂MnTiO₄ monoclinic 5.868 3.251 13.043 92.61 248.5 219

The elemental analysis of the materials obtained in the above Exampleswas performed by the ICP-AES method. Table 2 shows the results.

TABLE 2 K A B K A B K₂AB O₄ (at wt) (at wt) (at wt) (mol wt) (mol wt)(mol wt) K₂FeSiO₄ 30.6 21.6 11.4 1.93 0.950 1.00 K₂FeMnO₄ 27.0 21.6 20.11.89 1.06 1.00 K₂FeTiO₄ 26.3 19.9 16.8 1.92 1.02 1.00 K₂MnSiO₄ 31.4 23.911.8 1.91 1.04 1.00 K₂CoSiO₄ 32.3 22.3 12.1 1.91 0.878 1.00 K₂FeGeO₄27.6 20.1 25.2 2.03 1.04 1.00 K₂CuSiO₄ 31.5 24.9 10.8 2.11 1.02 1.00K₂CoTiO₄ 27.9 19.9 17.7 1.94 0.916 1.00 K₂CoGeO₄ 26.1 21.9 28.2 1.720.957 1.00 K₂NiSiO₄ 25.8 14.7 9.7 1.91 0.726 1.00

Test Example 1 Examination of Potassium Extraction/Insertion (Li HalfCell)

To perform charge-discharge measurement, K₂Co²⁺Si⁴⁺O₄ obtained inExamples 5-1 and 5-2, polyvinylidene fluoride (PVDF), and acetyleneblack (AB) were mixed in an agate mortar in a mass ratio of 85:7.5:7.5.The resulting slurry was applied to aluminum foil (thickness: 20 μm),which was a positive electrode current collector, and the resultant waspunched into a circle (diameter: 8 mm) to obtain a positive electrode.Further, pressure bonding was performed at 30 to 40 MPa so that thesample was not separated from the positive electrode current collector.

Metal lithium punched with a diameter of 14 mm was used as a negativeelectrode, and two pieces of porous Celgard 2500 cut out with a diameterof 18 mm were used as separators. An electrolyte solution (KishidaChemicals) in which LiPF6 was dissolved as a supporting electrolyte at aconcentration of 1 mol dm⁻³ in a solvent containing ethylene carbonate(EC) and diethyl carbonate (DEC) in a volume ratio of 1:2 was used. Thebattery was produced in a glove box in an Ar atmosphere because metallithium was used and any mixing of water with the electrolyte solutionthat happened became a factor of increasing the resistance increment. ACr2032 coin cell was used as a cell. Constant-current, charge-dischargemeasurement was performed using a voltage switching device by settingthe electric current to 10 mAg⁻¹, the maximum voltage to 4.8 V, and theminimum voltage to 1.5 V, and by starting with a charge. Thecharge-discharge measurement was performed with the cell in a 55° C.thermostat bath. As a result, the charging capacity of potassium ionspresent in the K₂CoSiO₄ structure was about 160 mAhg⁻¹ in the initialcharge process. This corresponds to about 1.4 electrons. Specifically,the reaction generated in Test Example 1 is as follows.K₂Co²⁺Si⁴⁺O₄↔K_(0.6)Co^((2+1.4)+)Si⁴⁺O₄+1.4K⁺+1.4e⁻(160 mAhg⁻¹)

Test Example 2 Examination of Cation Extraction and Insertion (KHalf-Cell)

The test was performed as in Test Example 1, except that K₂Mn²⁺Mn⁴⁺O₄obtained in Example 13 was used, potassium metal was used as a negativeelectrode, and the electrolyte used was obtained by dissolving KPF₆ as asupporting electrolyte in propylene carbonate (PC) at a concentration of1 mol dm⁻³. FIG. 62 shows the results.

Test Example 3 Examination of Potassium Ion Extraction/Insertion(Potassium Ion Secondary Battery)

The CR2032 coin cell shown in FIG. 63 was used as a cell. A Li box wasused as a negative electrode. Potassium bis trifluoromethane sulfonylimide (KTFSI) was used as an electrolyte, and an electrolyte solutionobtained by dissolving KTFSI at a concentration of 1M (mol dm⁻³) in apropylene carbonate (PC) solvent was used. Constant-current,charge-discharge measurement was performed using a voltage switchingdevice by setting the electric current to 10 mAg⁻¹, the maximum voltageto 4.2 V, and the minimum voltage to 1.5 V, and by starting with acharge. The charge-discharge measurement was performed with the cell ina 55° C. thermostat bath. In order to avoid the influence of the airexposure due to the hygroscopicity of the potassium-containing compound,the means for producing a battery etc. was performed in a glove box inwhich an Ar atmosphere was maintained. It was revealed that thehygroscopicity of the positive electrode compound was higher in theorder of Fe<Mn<Cu≤Co<<Ni, and that the hygroscopicity of apolyanion-based compound was higher in the order ofMnO₄≤TiO₄<SiO₄<<GeO₄. As a result, the initial charge capacity at acurrent density rate of C/20 was about 120 mAhg⁻¹ (52% of thetheoretical capacity). FIGS. 64 to 66 show the results. Further, FIG. 67shows the X-ray diffraction patterns of K₂Fe²⁺Mn⁴⁺O₄ before and aftercharge-discharge test.

REFERENCE SIGNS LIST

-   1. Potassium ion secondary battery-   2. Negative electrode terminal-   3. Negative electrode-   4. Separator impregnated with electrolyte-   5. Insulating packing-   6. Positive electrode-   7. Positive electrode can

The invention claimed is:
 1. A potassium ion secondary battery positiveelectrode active material comprising a potassium compound represented bygeneral formula (1): K_(n)A_(k)BO_(m), wherein A is positive divalentmanganese, iron, or copper; B is positive tetravalent silicon, germaniumor manganese k is 0.6 to 1.5; n is 1.5 to 2.5; and m is 3.5 to 4.5. 2.The potassium ion secondary battery positive electrode active materialaccording to claim 1, wherein the potassium compound has at least onemember selected from the group consisting of a cubic structure, atetragonal structure, an orthorhombic structure, and a monoclinicstructure.
 3. The potassium ion secondary battery positive electrodeactive material according to claim 1, wherein the potassium compound hasa mean particle diameter of 0.2 to 200 μm.
 4. A method for producing thepotassium ion secondary battery positive electrode active materialaccording to claim 1, the method comprising a heating step of heating amixture containing potassium; manganese, iron, or copper; silicon,germanium, or manganese; and oxygen.
 5. The production method accordingto claim 4, wherein the heating temperature in the heating step is 600to 1500° C.
 6. A potassium ion secondary battery positive electrodecomprising the potassium ion secondary battery positive electrode activematerial according to claim
 1. 7. The potassium ion secondary batterypositive electrode according to claim 6, further comprising a conductivematerial.
 8. A potassium ion secondary battery comprising the potassiumion secondary battery positive electrode according to claim 6.